Which Pumpkinrot monsters do you want to make? He has sentinels, groundbreakers, witches, ghosts, scarecrows of various types, and so on. Mastering the groundbreaker is probably the place to start first. This is one of Rot's more recent
groundbreakers. And this is one of his
older ones. Spookyblue has a
guide for the groundbreaker.
This is a good
guide for getting that crinkly corpsey look that Rot is known for. If you want to put some hands on your groundbreaker, SpookyBlue has
2 guides for those done in a Rotlike vein.
I need to take my own advice, but you're going to need to learn by doing. As I've hopefully demonstrated to you, Pumpkinrot took a few years to come into his own.