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 2guides 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.
Oh heck yes! Thanks so much for posting the links to Spooky's tut. I've been wondering how Pumpkinrot does his stuff as I want to make a large version of his Roots scarecrow crossed with his sentinel that has the corrogated tube for a nose/mouth.