hat you need:
10 gallon aquarium tank
Small or medium sized plastic "critter cage". Using the plastic critter cages allows for the housing of multiple clutches of eggs in the incubator that you might not want to get mixed up. The snap-down vented tops for these cages keeps the hatchlings from making an unexpected.
2 aquarium thermometers and one humidity gauge
1 small fully submersible aquarium heater w/temp gauge reading control. You want the fully submersible type that can be laid down on the bottom of the 10 gallon tank
Substrate of choice (sand, peat moss, vermiculite, potting soil, aspen bedding, etc.)
Top for 10 gallon tank (revised) -- I now use a plastic 10 gallon "critter top" available at most pet stores that has the center flip door and places on the sides for suction cups. You may also need to obtain some soft green dense spongy foam - the type of foam you can buy at a craft store for making seat cushions.
Small rock or brick (optional)
Setting it all up:
Place about 3 inches of warm water into the 10-gallon tank. Set the aquarium heater on the bottom of the tank (either to the front or the back of the tank). Set the heater dial for the lowest setting (usually 75 degrees F). Place one of the aquarium thermometers into the water so that you can keep an accurate idea of the water's temperature. Allow the water to stabilize for an hour or two. Adjust the heater dial, as needed letting the water temperature stabilize each time until you obtain a water temperature range between 80-84 degrees F.
Fill your small or medium critter cage about halfway with your chosen substrate. The substrate should be damp, but not soaking wet. When using aspen or peat moss I like to put the bedding into a bowl of water for 15-20 minutes to thoroughly soak up the water. Then I will take a handful out and squeeze the excess water out of it before putting it into the critter cage. My personal preferences for substrates are aspen bedding or peat moss. I tend to stay away from vermiculite - not that it isn't a good substrate, I just can't stand it. It has a slimy feel to it when wet and even my critters do not seem to care for it.
Place the other thermometer halfway down into the substrate. This will help keep track of the general temperature the eggs are experiencing while sitting on the substrate. Make small indentations with your fingers to create a cradle to place your eggs in so they do not roll around. Do not cover the eggs up as you will want to be able to see if any eggs go bad so they can be easily removed. Set the eggs into the substrate and put the top on the critter cage. It is important to keep reptile eggs in the same position that they were laid. Eggs that are heavily disturbed from being turned may cause the embryo to perish.
I like to use the round stick on humidity gauges that you can buy from the reptile section at the pet store. I stick it on the inside of the tank at the back centered near the top of the tank (see photo of setup; the glare from camera flash made it a little hard to see clearly).
When you have the water in the 10-gallon aquarium stabilized to your satisfaction, place the critter cage into the 10-gallon tank. If the critter cage tries to float, set a rock or brick on top to hold it down. Once your eggs are in the incubator, place the top onto the 10-gallon tank.
Notes about the top for the 10-gallon tank. I used to have problems with condensation dripping down onto the eggs when I originally used a plexi glass top with holes drilled in. I have since revised what I use for a top to the 10 gallon tank in the form of the plastic "critter cage" tops you can now purchase from a pet store. These tops, however, are vented which allows the humidity from the incubator to escape. I have found that spongy green foam from a craft store sewn (tied down to) inside the cage top works great. It keeps most of the humidity in while still allowing for some air exchange within the tank so the eggs do not mold.
It is important to take temperature readings from both thermometers once or twice a day and adjust the aquarium heater as needed to keep the temperature within the incubator safe for the eggs. Keep in mind that if the incubator is kept in a naturally warm (or cold) room the temperature of that room will affect the temperature of the incubator. Also, keep a close eye on the dampness of the substrate being used. If it starts to dry out it will have to be carefully re-dampened without getting the eggs unnecessarily wet.
Remove any bad eggs once you know that they are no good. Some small indenting of the eggs is normal for freshly laid eggs that have not yet expanded. Dents in the eggs can also be an indicator of low humidity levels. Some mold on the eggs is normal and is not always detrimental for the egg's well being. An egg that has gone bad has fully collapsed in, is all black and slimy, and/or is foul smelling.
This may or may not apply: Thin shelled eggs may mean that the mother did not get enough calcium during the gestation period. Giving the mother calcium supplements during the gestation period is very critical if the mother is prone to these types of eggs. Some people like to separate clumps of eggs that are stuck together. This is usually something that does not need to be done. If the eggs have thin shells and are stuck together do not try to separate them or you might tear them open. Typically you can only separate normal shelled eggs safely and then it usually must be done within a few hours of them having been laid.
Keep a clipboard with a chart to track all information regarding your incubation of the eggs (daily temperature readings, condition of the eggs, adjustments made, etc.). This information will help you in the future should you make incubating reptile eggs a habit. If all goes well, this will hopefully be the sight that greets you one afternoon: