Gather 3 Wither Skeleton Skulls
Gather 4 Soul Sand or Soul Soil
Go to the Nether or another open area
Place 4 Soul Sand or Soul Soil in a T shape
Put 3 Wither Skeleton Skulls on top of the upper row of the T
Make sure the last skull is placed to trigger the Wither spawn
Move away immediately after the Wither appears
Prepare weapons, armor, and healing items before spawning the Wither