Find a mushroom in a dark area, swamp, cave, or Nether
Collect the mushroom by breaking it
Place the mushroom on dirt, grass, podzol, mycelium, rooted dirt, or nylium
Make sure the light level is low enough for the mushroom to stay planted
Use bone meal on a mushroom if you want to grow a giant mushroom
Build enough open space around it for the giant mushroom to grow
Repeat with red or brown mushrooms as needed