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Wither Skeletons are three blocks tall, so you can walk under the beams but they can't. When it stops at a beam, walk up to the wither skeleton and hit it until it dies. Make sure to be on the opposite side of the beam to the Wither Skeleton. These are usually bones or coal, and sometimes a stone sword. Very rarely you will get a wither skeleton skull and it is used to spawn a wither boss.
Make beams two blocks above the ground in a nether fortress. When a wither skeleton comes, it will see you and walk towards you. Pick up the drops.