Find a mushroom in a dark area, swamp, cave, or the Nether
Break the mushroom to collect it
Place the mushroom on mycelium, podzol, or a block with low light
Keep the area dark enough for it to stay planted
Use bonemeal on a mushroom to grow a huge mushroom
Make sure there is enough empty space above and around it
Use red mushrooms or brown mushrooms for different huge mushroom types
Build a mushroom farm in a shaded room or underground
Harvest the grown mushrooms by breaking the huge mushroom blocks
Replant mushrooms to keep the farm going