@percy Make an outline to break the project into a series of subprojects of manageable size. Don't worry if your writing deviates from the outline; just adjust the outline as necessary.
And finally, you will have to revise several times over. If you can get someone knowledgeable to read it and point out where it's obscure, that's golden.
If you can find a word-processor that's compatible with software-style revision control, that can be good too.