Mine 3 Wither Skeleton Skulls from Wither Skeletons in Nether Fortresses
Collect 4 blocks of Soul Sand or Soul Soil
Place 3 Soul Sand or Soul Soil blocks in a T shape
Place 1 Soul Sand or Soul Soil block directly under the center block
Put the 3 Wither Skeleton Skulls on top of the three upper blocks
Step back and let the Wither spawn