First of all depending on the species you are growing incubation may not always be necessary. For Cubensis incubation at normal room temperatures is fine and temperatures in the 80's F may allow competitor molds to get a foothold allot more easy so generally incubators are not advised. Below is one way of making a incubator.By using the search function in the forum of this site you will find many other examples.
Try making an incubator out of two plastic boxes (about 1 by 2 by 1.5) and a 100 watt adjustable and submersible aquarium heater. Put the heater inside of box one, fill box one with water, put box 2 on top of the water and push it down so that the water rises up near the edge. I use a belt to strap box 2 in place (careful...water will get everywhere)....now let it run for a couple of days...adjust the temperature to MAX 82F degrees (don't rely on the aquarium heater's thermometer...put one of your own in the incubator)I don't let it go over 82 because higher temperatures drastically slow myc. growth and provide a better climate for competitor fungi and yours!. Good Luck!