Easy Squeezy Lemons. Kill a Wither Skeleton and hope it drops it's skull, it's a stupidly low chance, but Looting III will get you the head quicker. Once you've robbed the downed enemy of his once sentient head, don his head as a victory prize and watch all who lay eyes upon it quake in fear...or you can just kill another Wither Skeleton while wearing it to get an achievement 🤷♂️