In a previous post, I tried to convince you to write an open text. Supposing you agree, here is my number one piece of advice to help you get started: Start writing.
Pick a topic/section/chapter that you'll eventually want to cover - pick a favorite! - and start writing that portion of the text. I wrote two sections of a math book. An "equivalent amount", depending on your discipline, should be enough to serve your purposes. And what, pray tell, are these purposes?
First off, they serve as play. You want to write, so do it! Don't impose too much structure on yourself. Just have fun producing something. In the end, you'll change much of what you just wrote. But you'll develop skills you'll need later (just like your play as a kid did).
Secondly, this play will point out things you'll need to develop in the future in a way that is more meaningful than anything I could write/advise. I wrote a math book and thought I knew how to make good graphs of functions. Then I made two. And realized how different they looked. And how horrible it would be if each graph in the text had its own look. My play introduced my need for graphical standards. (I could tell you to "develop graphical standards", such as font sizes and color schemes. But your playtime will make clear what things need to be standardized.)
Finally, undertaking a big project is hard. By writing something - anything - you will have started. Keeping that momentum going is easier that starting it. If the first few weeks are spent trying to plan, after a few weeks you'll likely be discouraged for you'll have little to show for your work. Write something. Tell people you have a section already completed. Then step back and do the necessary planning. The next posts will cover some of those planning needs.