The easiest way to do this is to kill a Wither Skeleton to obtain a Wither Skeleton Head then equip the head and kill another Wither Skeleton.
Unfortunately, the drop rate is very low at 2.5%, meaning it's likely you'll have to kill 40+ Wither Skeletons until one drops a head. It could happen on your 1st kill or on your 100th kill, it mostly depends on luck.
However, the looting enchantment increases the odds of a Wither Skeleton dropping a head by 1% for each level (up to a 5.5% chance at looting level 3).
Hope this helps!