Obtain nether wart blocks or nether wart items
Place nether wart on soul sand
Keep the soul sand in the Nether or in a suitable growing area
Wait for the nether wart to grow through its stages
Harvest it when fully grown
Replant harvested nether wart on the same soul sand
Use bone meal only if your game version or mod allows it
Light level does not affect nether wart growth
Keep the area safe from mobs and accidental trampling
Expand the farm by placing more soul sand and planting more nether wart