Nolo Press, which is known for putting out do-it-yourself legal books, has a book for writing one's own patents [0]. At the link, there's a sample chapter.

They also have a number of pages on their website related to this topic, which might be worth reading first[1].

[0] https://store.nolo.com/products/patent-it-yourself-pat.html

[1] https://www.nolo.com/legal-encyclopedia/patent-law