Find a name tag in dungeon chests, mineshaft chests, stronghold chests, woodland mansion chests, ancient city chests, or fishing
Trade with a librarian villager for a name tag
Use an anvil to rename the name tag if desired
Right-click or use the name tag on a mob while holding it
Use an anvil with a name tag and an item to rename the mob through the tag
Use a name tag on a mob to prevent it from despawning