Find a mushroom in a dark area, swamp, cave, or Nether
Collect a red mushroom or brown mushroom
Place the mushroom on a block with low light level
Use mycelium, podzol, or nylium for easier placement
Keep the area dim enough for the mushroom to stay planted
Wait for it to spread naturally to nearby suitable blocks
Use bone meal on a mushroom to grow a giant mushroom
Make sure there is enough space above and around it for the giant mushroom to form
Harvest the giant mushroom blocks with an axe or hand
Replant mushrooms and repeat the process