Find a name tag in a dungeon chest, mineshaft chest, stronghold chest, fishing, or trading
Get an anvil
Place the name tag in the anvil
Type the name you want on the name tag
Take the renamed name tag from the anvil
Hold the name tag in your hand
Use it on a mob to apply the name
The mob will keep that name permanently
Use it on a mob you want to rename
Name tags do not work on players
Name tags can be used on most passive, neutral, and hostile mobs
Use a name tag on a mob to prevent it from despawning in many cases
Use a name tag on a mob to give it a custom name for identification or fun