mushrooms are essentially brown button mushrooms (crimini) grown to maturity. Growing them from scratch involves six major steps. In a nutshell:

1. Make the composted substrate upon which the mushrooms will grow. Typically the base material for the compost is horse manure mixed with straw. After adding water, nitrogen, and gypsum, the material is stacked and allowed to compost. The process produces heat, ammonia and CO2 - all things you don’t want in your home.
2. Finish the compost. This step removes excess ammonia and kills any pests that may remain. Air moves through the compost to carry away ammonia (again something you don’t want in your home) and the compost is held at around 125–130F to kill off the pests.
3. Add spawn. Spawn is living mushroom mycelium that will eventually spread throughout the compost and produce the mushrooms you want to pick. Making spawn is another process that requires considerable skill.
4. Casing. This is typically a mixture of peat moss and ground limestone. It is placed atop the spawned substrate like a blanket. It holds water and gives the mushroom a place to start fruiting.
5. Pinning. This is the start of the fruiting process. The tiny mushrooms begin to grow and expand from barely a dot to the mushroom you expect to see. To get this started you need to get fresh air to your mushrooms so that CO2 level drops below 0.08%. It takes around three weeks for this step.
6. Harvest. The mushrooms are typically picked over a 3 to 5 day period then left to rest a few days when they are picked again. This can be repeated for a month or more.

This works if everything goes well. There are a lot of mushroom diseases that growers have to manage and they, also, are not something you want in your house.