Find a mushroom
Collect a mushroom with your hand or any tool
Place the mushroom on mycelium, podzol, or a block with low light level
Keep the area dark enough for mushrooms to stay planted
Use bonemeal on a mushroom to grow a huge mushroom
Harvest the huge mushroom for more mushrooms
Build an indoor mushroom farm with low light and a suitable floor
Spread mushrooms by placing them near each other in a dark area
Use water channels or hoppers to collect harvested mushrooms
Repeat harvesting and replanting to keep the farm producing