Use 12-inch mini-bales of hay or straw -- available at feed or craft stores -- for ease in sterilizing them. Choose a site out of direct sun for hay-bale gardens, whether they are located inside or outside.
Immerse the bales in boiling water in a deep roasting pan or a large water-bath canner. Remove the bales with tongs once they're saturated. Place the soaked bales on baking racks and allow them to drain. Cool the bales to room temperature.
Place the bales directly on the ground, or on tables or trays, for ease in maintaining and harvesting them. Wash your hands thoroughly, then open pockets in the hay on the top and around the sides of the bales.
Insert oyster mushroom spawn into the open pockets in the hay. You can purchase the spawn online or from local fungi farmers. Ensure good contact by pressing the spawn into the bales.
Pull the hay around the mushroom spawn. Close the pockets by pressing loosened hay into the bale and tamping it.
Water the bales, preferably with water from rain barrels or filtered water. Keep the bales constantly moist but not soggy. Harvest the mushrooms at any stage of growth.