You will need ventilation, a thermostat, and as for the egg cartons, if you are using the foam ones, I'd suggest cutting vent hole in them so the eggs can breath. You will need some way to rotate the eggs as well. Such as an egg turner. (You can make these easy enough, or just use something like a thick book moving it from one side of the egg carton to the other 3-4 times per day for the first 18 days for chickens, or 25 days for turkeys.) I highly recommend an automatic egg turner.
A thermometer is a must for regulating the temperature inside the incubator. (The thermometer doesn't do the regulating, but it gives you the information you must have in order to make changes in the thermostat settings.) Really 2 thermometers is better, and even a second heat source as a back up in case the first one stops working.