Purchase a mushroom-growing kit or pack. Many different edible mushroom kits are available, including button, Portobello and French mushroom packs.
Read the kit's instructions to familiarize yourself with the pack you purchased. Most kits consist of a growing box or container with a nutrient-dense substrate, such as saw dust or soil. The mushroom spores are already introduced to the growing pack. You must add a specific amount of water to the kit to initiate growth, then place the kit in a location where it will be able to maintain proper humidity. A laundry, utility or bathroom are all locations that generally retain humidity well.
Monitor the mushroom kit daily. Water it precisely as the instructions specify; too much water will destroy the mushrooms, but too little will prevent growth. Small white or brownish growths on the substrate indicate that the mushrooms are growing and that your current maintenance and setup is working.
Harvest the mushrooms when they are full-sized. The instruction pamphlet included with the mushroom pack will likely specify a time-frame for harvesting, as well as size estimates. Store the mushrooms in the refrigerator to keep them fresh.