Go to the Nether or Overworld and gather the materials
Collect 3 Wither Skeleton Skulls
Collect 4 Soul Sand or 4 Soul Soil
Place 4 Soul Sand or Soul Soil in a T shape
Put 3 Wither Skeleton Skulls on top of the T shape
Make sure the area is clear enough for the Wither to spawn
Wait for the Wither to fully appear and activate