This can take anywhere from 10 to 30 minutes to complete, depending on crafting luck.

To farm worm heads, run back and forth between these two large worm spawns in Iron Hammer Mine:


The worms have fixed respawn timers of 20 seconds and an 80% drop rate for worm heads, meaning you can farm an average of 48 heads in 10 minutes. Only the large worms drop heads, so you can ignore the small ones.

The catch is that each head only has around a 6-8% chance of converting to a skin at the cook, and you need 5 skins for the recipe, so it's possible you could need 100+ worm heads if you get bad luck.