Purchase sterilized mushroom spores and the medium in which they grow the best. For instance, shiitake mushrooms grow best in hardwood or hardwood sawdust, while white button mushrooms grow best in composted manure.
Fill a garden flat halfway with the selected growing medium such as hardwood sawdust or composted manure. A garden flat is a plastic tray used to plant seedlings. Use a flat that does not contain dividers.
Raise the temperature of the growing medium in the flat to about 70 degrees Fahrenheit. You can try placing the flat in a warm area, or place a heating pad underneath it.
Place the mushroom spores onto the growing medium. Cover them with a layer of sterilized topsoil about 1 inch deep.
Water the soil to get it damp. Spray the soil two to three times weekly to keep the soil damp. Do not overwater, as you can inhibit the mushrooms' growth or even kill them.
Move the flat to a dark, well-ventilated area where the air temperature is between 55 and 65 degrees Fahrenheit.