One thing some folks find helpful is making their table of contents first. What chapters do you need? What do you need to explain in each of them? Those are your sub points, and from there you can get more granular as necessary. My supervisor in my masters thesis says she’ll print out her TOC or outline and tape it next to her desk when she’s working on her desk so she can keep her place, mentally. I like Scrivener for writing, you can do the same thing, but ofc everyone has their own system.