Just doing as MegF. says probably works fine
But I do use a homemade one:
Get an aquarium (10 gal will be fine unless you have a ton of eggs), a submersible heater (the kind that have a temperature control), and some way to seal the top. Not airtight, but enough to keep the humidity and warmth mostly in. Fill the tank partway with water (about 1/3 the height) and submerse the heater. Set it to 82F (and make sure the area you set the tank in doesn't get above this!).
Put a smaller waterproof container inside, ie a critter keeper or plastic shoebox, and put the eggs inside that (in vermiculite or whatever substrate you like to have eggs in). The inner container can either be on a small platform (I use bricks
) or weighed down a bit (gravel). Try to find a lid that won't drip on the eggs (maybe stretch a cloth over the second inner container).
It ends up looking like this:
http://www.spiffyspace.com/photos/snakes/eggs/incubator20050427.jpg
http://www.spiffyspace.com/photos/snakes/eggs/incubator20050427b.jpg
Check periodically to make sure the water level covers the thermometer, the temps are okay, and the eggs look okay. I usually remove rotting slugs although some say it's not necessary (just apply gentle but constant pressure and they should slowly peel off). If you see any mould or fungus you can get a q-tip and swab it off with a bit of mouthwash
This is what I do and it's worked out pretty great so far!