Start anywhere you feel like you want to start.
Assign a character for to-do's, like ? so when you get to a part you don't feel like writing go (?protagonist monologue about ww3)
Then later on search back in your document to ? and fill in the gaps.
Don't edit yourself after each chapter, you'll only start doubting your own work. Instead edit twice when your done with your novel.
Don't be afraid to rewrite sections or retcon yourself when needed, but not after each chapter.
Let someone proof read it regularly along the way. Not for spelling/grammar checks but more along the lines of 'does this make sense?' when writing something complex especially in a sci-fi world it's easy to take certain things for granted and under explain them. When your proofreader finds things they don't quite understand, explain it to them in the novel.
If you have allot of high-tech gadgets, remind your readers what they do. If you explained how a warpgateprismthingy works in the first chapter, and during chapter 5 your story involves a warpgateprismthingy let the character have a brief 'flashback' to the first time you explained it, not for the character but so your readers can still follow along.
Make character bio's, no set form, just anything you're comfortable with (personally I make little Dungeon and Dragon sheets) and when in doubt you can check back to their bio's, as in "Is this really something Fred would do?" On those bio's you can also update their personal growth. Make a little memo on each card whenever they experienced something that changes them in some way, even minor.
Be aware of the God-character. Don't let your protagonist become too powerful, or present them with challenges you know they could overcome easily.
But most of all, just start. Start writing if you haven't already. It doesn't matter where, but start. Start at the final endgame battle if you must but just start.
Edit: Need coffee before I start typing in other languages.
