Find a brown mushroom in the Overworld, Nether, or other dark areas
Place the brown mushroom on a block with low light
Keep the light level low enough for the mushroom to stay planted
Use a 1-block-deep area or enclosed room to control light
Place mycelium, podzol, nylium, or another valid mushroom-friendly block if needed
Bone meal a brown mushroom on valid blocks to grow a huge brown mushroom
Ensure there is enough empty space above and around it for growth
Harvest the huge brown mushroom with any tool or by hand
Collect brown mushrooms from the giant mushroom blocks
Replant mushrooms to repeat the process