Pick the mushrooms you want to collect spores from and place them in a berry basket or similar collection container. You will have the best luck with fresh, mature mushrooms. You can use supermarket mushrooms, but these are usually too dry to produce a good print.
Rub alcohol on your hands and put on a pair of latex gloves.
Using your knife, remove the stem from small mushrooms. For larger mushrooms, slice off sections of the cap.
Place the cap or cap sections gill-side down on a piece of paper or glass. If your mushroom has white spores, black paper is best, and vice versa. Glass will accommodate all spore colors.
Place a bowl over the mushrooms to reduce air movement.
Leave your mushrooms for 12 to 24 hours. When you return and lift the mushroom caps, you will see millions of tiny spores on your paper or glass. It will look like a fine dust.