My first incubator was an aquarium filled with enough water to cover a submersible aquarium heater turned to 90degrees. I put the eggs in a separate container with moistened vermiculite floated it on top of the water and covered the top of the aquarium with plastic wrap. Out of 8 eggs 4 hatched. Not bad odds I think considering.
I know quite a few people have used old mini fridges and freezers for incubators. If you google Mini Fridge Incubator I'm sure there are a few guides. Here is one I found...
I am just going to offer some freindly advise. I was dead set on making my own incubtator, bacuse I wanted to save money. I had an aquarium type incubator set up, and everything. The trouble was that the temp kept jumping around, a little too much for my likeing. I looked into Hovabators, at lllreptile.com I belive the Thermal Air Veiw model (what you would want- not the fan) is only 40$ and comes w/ vermiculite and deli cups. I got it, and it works very well. I really suggest looking into spending a tad bit more money for a more reliable incubator.
I would have to agree with Jake that a hovabator would be a better bet. I doubt that you would save much money making your own. Plus i bet you will get a better hatch rate using the hova.
I have only used it once, but I took an old refigerator, a drop light from home depot, a 75 watt bulb and a zoo med thermostat and made an incubator for my bearded dragon eggs. I hatched out 8 babies from 12 eggs. I keep it in my reptile room ( with the a/c it stays about 78 degrees) and it stayed a pretty stable 84 inside the "freezer". The highest it reached was 86 once when I didnt have the air on, and lowest is about 82 when the thermo kicked on. I am looking for something smalle, a 24 cubic foot fridge is just a little more than I need!