Find a Nether Fortress in the Nether
Look for Nether Wart growing on Soul Sand inside the fortress
Break the Nether Wart to collect it
Use a Diamond or Netherite Hoe if you want to harvest it faster
Bring Nether Wart back to the Overworld if needed
Plant Nether Wart on Soul Sand to grow more
Trade with a Cleric Villager at higher levels for Nether Wart in some versions
Check Bastion Remnants only if playing a version or modpack where it can appear there