Summon or spawn the Wither
Lure the Wither to a location where it can kill mobs
Let the Wither kill a mob
Collect the Wither Rose that drops where the mob died
Use a mob farm or controlled enclosure to repeat the process
In Bedrock Edition, use the Wither to kill any living mob except the Wither itself
In Java Edition, use the Wither to kill any living mob except the Wither itself and the Ender Dragon