Adapt the lids of the Mason jars by replacing the supplied two-piece lids with a one-piece plastic storage cap. Drill a 1-inch hole in the center of each cap with a paddle drill bit.
Cut, with scissors, a circle of Tyvek 2 inches wider than the jar mouth, so that it overlaps the edge by an inch. You are adapting Tyvek, which is cheap and reusable, to fill the role of a filter disk, which is bought commercially. Similar to a filter disk, Tyvek allows air exchange while screening out contaminants.
Add a substrate to the Mason jar. Lay the Tyvek across the top. Screw on the plastic storage cap.
Sterilize the jar with substrate in boiling water or a pressure cooker. Allow it to cool overnight.
Loosen the plastic storage cap and the Tyvek covering, ideally within a specialized box that admits your hands but lessens the risk of room air circulation contaminating the substrate. Inoculate the substrate with mushroom spawn.
Lay the Tyvek back across the jar mouth and screw on the plastic storage cap. Place the jar in a dark closet or dresser drawer to allow the mushrooms to grow.