I started out by analyzing the whole series, diagnosing problems with plot, worldbuilding, character development and stuff like that. This is a picture of my binder with the printout of Daughter of the Wildings when I first started that process.
Here's the first page of Book 1 of Daughter of the Wildings, with lovely red scribbles, scrawls, arrows, lots of words crossed out, and some blue ink where I changed something and then changed my mind about the change. And this page is pretty clean compared to a lot of the pages I've done since then. The numbers down the sides are references to my notes; I have hundreds of pages of notes and thousands of individual revision notes for this series.
The great thing about revising this way, analysis - plan - markup, is you discover all the issues with your manuscript and figure out what to do about them before you ever start in with the red pen, so that you don't get halfway through and then *forehead slap* realize that subplot isn't working or this character's development is way off. I'm still making changes to my plan as I go, but it's easy to go into my notes and update them. I still refer to my notes as I do the markup; the summaries on the cards give me an idea of the major changes that have to be made. I do those, then go back and check on the more detailed changes that I put in my notes. Half the time, those changes don't apply any more because I already re-wrote that whole section while making the big changes.
Also, I haven't had the reveal for the cover art for Book 6 yet, so watch for that in a week or so!
(Note: I revise using the method taught in Holly Lisle's How To Revise Your Novel online course. That's my affiliate link, and I'm an affiliate for that course and recommend it every chance I get because if you want to publish your writing, whether self or traditional, it's the best $250 you can spend on your writing.)