First Time Modder, Asking about PatchOperations

[jessiepums]

I'm trying to add the the ability to make skull spikes stuffable with stony materials, while also giving them a morbid meditation type.

I'm stuck on how exactly to select those parts/lines to add onto.

<xpath>/Defs/ThingDef[defName=Skullspike"]/stuffCategories/li/

This is basically how far i've come to understanding, any slightest bit of help or nudge forward is appreciated.

I know this is probably the least coherent question but I just jumped into all of this less than a day ago.