Minecraft wiki entry for Defeating_the_wither, associated text: In order to spawn the Wither, you need to acquire three Wither skulls. Wither Skulls are acquired by killing Wither Skeletons. In order to maximize your chances of acquiring wither skulls, it's recommended that you carry a sword with Looting on it. Even though the maximum drop chance is 5.5% (with Looting III), it is best to get the highest possible drop chance. The wither requires three skulls to build, so you may have to kill hundreds of wither skeletons to get them. However, the only other ingredient needed to build the wither is soul sand or soul soil, which is extremely common in the Nether.