Gather 3 Wither Skeleton Skulls
Gather 4 Soul Sand or Soul Soil
Go to the Nether or another safe area
Place 4 Soul Sand or Soul Soil in a T shape
Place 3 Wither Skeleton Skulls on top of the T shape
Wait for the Wither to spawn
Move away immediately
Gather 3 Wither Skeleton Skulls
Gather 4 Soul Sand or Soul Soil
Go to the Nether or another safe area
Place 4 Soul Sand or Soul Soil in a T shape
Place 3 Wither Skeleton Skulls on top of the T shape
Wait for the Wither to spawn
Move away immediately