@BioLucy I wrote a post about it last year - -these resources are computer-science oriented, but some of the general precepts I think cross over.
Something that really seems to help is to get another person to read it. When I was helping review SOPs (not now), some of them were ready to go, and some needed many revisions and more thought.
https://amytabb.com/tips/2021/01/18/statement-of-purpose-help/