I came across this page when a pixel artist I follow posted about it online a week or two ago. While it’s not something that really helped me at my present stage of my art “career”, it covers quite a few fundamentals that should be useful for others: https://pixeljoint.com/forum/forum_posts.asp?TID=11299
It’s a decade old, but still very useful.
Also, for ideas for what to include, I think the biggest topic that this tutorial should cover is anatomy and size/proportions. 99.9% of fully custom portraits that have issues with them are, to me, largely because the artist doesn’t seem to have enough of a grasp for how the features of the body and FE-style portraits are mostly structured, leading to portraits that frankly look worse than a splice would - bodies or arms too small, head too small/big, the head lacking volume and depth, etc. If the goal is to allow people to hone their artistic skills enough to forego using splices in their projects, then I feel like covering this is paramount to making that a reality. Understanding angles is probably right up there with anatomy and proportions, too.
(This is historically why it was often suggested for most people to start with splices so that they can learn these fundamentals through experience before starting into doing custom work and diving into the deep end without what amounts to a safety net. I understand wanting to be able to create the vision in your head, but it really helps to have some kind of base and foundation to start with before getting to that point.)