Make sure you do a lot of research before you decide to get into breeding your geckos! You must be positive that you have a back-up plan in place in case you run out of room to keep all of your hatchlings. Gecko babies are not as easy to rehome as kittens or puppies - many animal shelters will not take them, and most pet stores are not allowed to buy stock outside of their predetermined sources. Read up on how to become a reptile seller or trader if that is your goal!
It is also a breeder's job to think about the ramifications of their breeding projects. If you own a sub-par or even defective gecko (perhaps your gecko's crests are very small, or it has an overbite or underbite), I recommend that you think twice about breeding that animal. It can still be a wonderful pet, but all pets do not need to be bred. The gene pool in captivity is regulated entirely by you and your fellow breeders - as long as we are "playing God" with their genetics, we should be mindful of possible outcomes. Would you want to buy a sub-par gecko for a future breeding project? If you wouldn't, why would anybody else? Make sure that any geckos you plan to breed are healthy and excellent representatives of their species.
Weight recommendations vary from breeder to breeder. In the past, I have encouraged females to not be bred until they are holding a steady weight over 35 grams. Waiting longer to make sure an animal has basically stopped growing is encouraged as well - but please do not equate more weight with better health. Cramming a ton of weight onto these animals' tiny frames can cause issues with reproduction, as well as other issues which affect obese animals of any species. Some crested geckos can reach 70 grams or so with no "fat rolls" as their bodies are built longer than others, while some animals can start showing fluffy obese rolls at 45 grams. So - I recommend that a female gecko weigh no less than 35 grams and be 2 years old or more (keep in mind however that geckos do grow at different rates depending on diet, temperatures, and genetics so different breeders approach things at their own speed) before being introduced to a male for breeding. The male should be around the same size as the female, though in many cases the male being slightly smaller has not caused problems during breeding. The male does, however, need to have enough mass to be able to grip the female.
Whether you introduce the male to the female for a night, a week, or leave them together for the season is up to you as a breeder. Understand that if you choose to leave them together, as mentioned previously, every new set of jaws in an enclosure is a new potential set of issues. Keep an eye out for bite marks and weight loss in both animals.
Sometimes a male and female might seem uninterested in each other, but often, mating takes place at night when you're sleeping. During copulation, a male will grab the female by her crests with his mouth and will make contact to her vent with one of his hemipenes. You may hear some "squeaking" sounds as they vocalize to each other. This act looks rough, and sometimes a gecko may receive bite-marks from mating, but they generally heal well if the enclosure is kept clean.
Females are not always receptive to the advances of a male. If a female is not receptive, as the male begins to sniff her and touch her, she may shake her head and/or tail back and forth quickly, and utter an "eh-eh! Eh-eh!" sound. More forceful females will bite a male to discourage him, so this is something else to keep an eye out for. You can always remove the male and place him back with the female in a week or so to see if she is receptive.
Don't be afraid if you see something red or pink hanging from the male's vent after copulation. This is the hemipenis, and he will often lick himself until it fits back into the vent. If you see that your gecko is having trouble getting his hemipenis back into his vent (a couple of hours after copulation), keep an eye on it and try to help keep it from drying out - some have recommended using sugar water or honey. You may need to call your exotics veterinarian if your gecko has issues re-inverting his parts.
If the mating was successful, your female will be gravid - not pregnant, as pregnancy indicates a live-bearing animal, not an egg-layer. About thirty days, give or take, after successful copulation, a female will look to lay two eggs in a soft substrate. If you are making use of a paper towel substrate instead of a bioactive enclosure, you will want to provide the female with an egg-laying box. In my experience, it's a good idea to provide a lay box for any female over about 20 grams, as they can lay infertile eggs as well, and the box gives the gecko a comfortable enclosed space to dig and lay. I prefer to use a wide Gladware or Ziploc type plastic container with many ventilation holes and a little hatch for the female to crawl into. You can use many different kinds of substrate in this laybox, though I prefer a mix of very slightly moist sifted sphagnum peat moss and organic soil - if it still clumps together a lot, it is too wet - "just right" feels damp but still fluffy.
The female will often use her head and limbs to shovel as she creates a "nest" to lay in. She may spend all night, or even several nights, digging and burying herself in the substrate before she actually lays. Eventually she will bury the eggs in the substrate. Don't worry if you see your gecko fully or partially submerged in the soil - they can generally breathe quite well even if their heads are covered in the substrate. Pulling your female out if you see her head-first in the soil can disrupt her laying schedule.
I have read several sources advising to stay away from coco fiber/coir as a laybox and incubator substrate. At best, the material may stain the eggs, and it is thought that there may be a chance that the materials may leech into the eggs and harm them. I have not experimented with coir, so I cannot confirm or deny this claim, but I prefer to play it safe.
After eggs are laid, you may notice that your female has a bit of a "deflated" look anterior to the rear limbs, where she may have been a bit pear-shaped before, so that is something else to keep an eye out for if you're unsure if your gecko is gravid.
A healthy female can lay a clutch of two eggs every thirty days or so. Viable eggs will generally look bright white all around - though if your eggs come out looking a little yellowish, it's recommended that you don't throw them out unless you're 100% sure they're infertile. Eggs can be incubated in a separate container; I prefer to use aquatic planting soil or APS as an incubation material. This is a lightweight rock-like material which looks like aquarium gravel, but which actually holds water as it is porous. It has the added benefit of changing colors as it gains and loses moisture, so you can tell at a glance whether you need to add more humidity. The APS should be rinsed thoroughly and boiled ahead of time, and then allowed to sit in a sieve for a few minutes until it is cooled and no longer dripping. Aquatic planting soil can be found at many plant nurseries where koi pond supplies are carried. Use your finger to make indentations in the APS that will fit the eggs. The eggs can be set inside so they are one-half submerged beneath the incubation material. An incubation container can have tiny holes punched into it to provide air for the eggs and eventual hatchlings, or, if you prefer to follow my methods, you can try it without the holes and just open the lid once a week for a few minutes to vent in new air. Just be wary of checking the container often once you know incubation time is nearing an end, so you can catch hatchlings and make sure they have enough air.
Gecko eggs are not like birds' eggs. They do not need to be rotated; they should be left with the same side facing up in their incubator through their entire incubation period. You can prevent shifting by making a small pencil mark gently on the top surface of the egg. In my experience, eggs will still hatch if you have an "accident" and they get bumped, so please do not panic if that happens; just keep in mind that in nature, the eggs would be buried under soil and left facing one way, so it may be developmentally helpful to keep from moving the eggs too much.
Incubation can go from around 65 days at room temperature, to all the way up to 120 days and beyond, depending on your temps. If you keep your home on the cooler side, your eggs may take longer to hatch, but this is thought to be beneficial to the growing embryo, as it has time to absorb all of the egg's nutrients and may be stronger when it does finally hatch. In my experience, eggs incubated on the hotter side, at around 76 F+, seem to hatch out smaller with less-developed crests and tend to be a bit more flighty upon hatching. I find that about 70 to 72 F is my ideal incubation temperature.
You can check to see if your eggs are viable by "candling" them. Take the egg into a dark room and hold it up to a very bright LED-type light (the light on most cell phones works well for this). If you see a small red circle at the top of the egg, chances are, the egg is good. This red circle will develop quickly as the first week goes on, so if you didn't see anything the first day, try again in a few days. Keep in mind that the less you handle the eggs, the better, if for no other reason than to avoid accidents, so taking them out every day to candle them isn't a great idea.
Generally, the two eggs in a clutch may hatch within an hour to within a day of each other. However, it is not unheard of for a clutch to hatch around a week apart.
Please note, often first-time laying females have poorly calcified eggs, or they only lay one egg instead of two. As long as she is at an appropriate weight, is feeding well on a good diet, and shows no signs of illness, this should not be a huge concern. Usually a healthy gecko will get it "right" around her second or third clutch. This is a touchy subject, but in my personal experience, I prefer to freeze (destroy) eggs which are laid with poor calcification. I have found that many eggs with roughly-calcified shells have a greater percentage of geckos which either cannot cut through the shell, or geckos that hatch small, weak, or with other congenital defects like MBD. I also do not heavily encourage the assisting of hatching - if an egg is meant to hatch, it will hatch. Again, this is my personal stance!
One problem to be aware of before attempting breeding is egg-binding, or your gecko becoming unable to pass eggs. If a female is undersized, or there are other health issues present including a problem with calcium, she may retain the eggs. The eggs may continue to grow and possibly fuse together inside of the female's body, making it difficult or impossible for the eggs to be passed. If your breeding female's sides start looking bloated, or she becomes very lethargic and ceases or slows in food consumption, keep a close eye on her, but do not bother her overly much, as that can stress her even more and prevent laying attempts. If you are expecting eggs and they are very long in coming, it may be worth your time to get your female checked at a reptile-knowledgeable veterinarian's office to find out if the eggs can be safely passed.
Once a gecko has developed fully inside the egg, it will break out using a pair of "egg teeth" developed solely for this purpose. Some have claimed that they are lost with the initial shed, but I have observed them (less noticeably) after this shed.
When hatchlings come out of the egg, they may stick their heads out and just rest for a while. Just let them come out on their own! They may take a few minutes or around an hour to fully emerge. Once they come out, be careful, as they can move very quickly. I keep two hatchlings of the same size in a small "shoebox bin" type enclosure, with very small branches and fake foliage for the first month. Their water and food dishes should be very shallow to prevent drowning, and their enclosure should be misted gently at night. While you can offer food right away, it has been my experience that most newly-hatched geckos do not show interest in eating for at least the first few days, so do not panic if you don't find food missing immediately. I personally do not recommend that you try to hand-feed the newly-hatched geckos, as it is incredibly easy for a small gecko to breathe in some of the food. Think about how things happen in nature - their parents would not bring them food. They will find the food on their own when they're ready! I encourage including live food in the diet starting at a week or two of age, as this will really jump-start their growth. Other than the smaller size of enclosure, their care is similar to an adult crested gecko.
A female gecko should be encouraged to take a break from breeding for a few consecutive months every year. One mating with a male can cause a female to lay fertile eggs for several months as their bodies can store sperm for some time, so if you want your female to stop laying, you may try a few different things. You may remove the male from her enclosure, attempt to bring the temperatures down to the high 60's, reduce the length of time your geckos receive light during the day, or a combination of these. The cooler weather and shorter daylight period will generally cause the gecko to stop laying eggs. Most breeders use the winter time when temperatures are naturally chillier to "cool" their geckos. During the cooling period, a female can rebuild stores of fat and calcium, and make her ready and healthy to go for the next season. Here in the north, my female geckos tend to stop laying in September, or October at the latest, without my interference, and generally pick up laying again in February or March of the following season. If you live in a warmer climate, you may need to take measures to make sure your females get adequate rest. You should not allow them to lay all year 'round, and if managing your daylight and temperatures to simulate a winter period is not possible for you, I heavily advise you not undertake breeding.
Crested gecko genetics are still being figured out; one thing that makes these animals intriguing is watching their offspring develop over time. Generally, pairing an animal that is showing a pattern or trait with another animal showing the same pattern or trait will yield a pretty good chance of offspring with that pattern or trait, but this is not always the case. In all breeding projects, do research ahead of time! Figure out if there is a desire for what you are planning on producing, and always be prepared in case you cannot find new homes for those which you are producing.