I'm trying to figure out how to make a custom spell that summons something but I don't really know where to start. Ultimately I was hoping I could make a custom grimoire with unique spells to create a hidden sub-class. Can anyone get me started?

There is a mod called "Al amir spellbook" or something like that, on the Nexus. It has a custom spell that involves summoning, if you were to look through those mod files I'm sure you'd learn something to get you started.

i'd start by copying a spell similar to what you want in the end, giving it unique IDs and tweaking it from there