>at this stage i have only a small idea of what's an improvement and what isn't.
You have my sympathy! I experienced something very similar on my first novel. I was just so very, very lost. I recall asking people how I was even supposed to know when the first draft was done? What does done look like when you know perfectly well you're not done?
It's been some years now, so memory gets garbled with later efforts, but here's a stab at advice.
I quickly lost perspective about high level and low level. I looked more at chapters; I've since narrowed that to individual scenes (defined as where a location changes or there's a major shift in action or characters, if location doesn't change). It's usually only a matter of pages, a couple thousand words to a few thousand.
At that level, do I feel (at this point I only listen to myself) that the scene works? Does it do what it needs to do?
If so, I look at the ends. Does this scene follow well from the previous? Does it lead into the next one?
Along the way, I make note of continuity problems, inconsistencies in voice. I usually make small editing corrections like spelling, but I try to keep from getting too involved there, in case I am looking at a rewrite.
What I didn't do the first time was make a note to myself as to whether this scene works or needs work. (they nearly all do). Failing to do that often meant making changes, then having to go back through to see what I forgot to do. *sigh*
In theory, I work through the whole thing. In practice, I'd start to spot larger issues. A relationship that needs adjusting. An inconsistency in character development across larger parts of the novel. I'd get distracted by that and start making changes. At a wild guess, I'd say I wrote 20% more of the novel during revision that either never made it into the final version, or was radically changed. What had been two pages of a new or changed scene might have wound up as little more than a comment by a character. That sort of thing. I was groping in the dark.
It's worth repeating that. Despite all advice, I was still groping in the dark because I had to figure out how *I* revised. Two factors propped me up (but I often despaired). One, as an academic, I'd written complete works before. I knew I could do it. I knew there would be flailing and wailing, but that if I just kept working, however inefficiently, I'd get there. Wherever there was.
The other factor was a kind of fear. I had said I would do this. Said it in what was very nearly a promise. The fear of not finishing outweighed frustration and confusion and uncertainty and self-doubt. I kept going because I couldn't *not* do it.
It sounds harsh, but my best advice to would-be writers is: if you can stop, you should. Writing is for those who can't seem to avoid it. To put it with more humor, though no more kindness, Dorothy Parker said the best thing you could do for the aspiring writer is to shoot them in the head while they are still happy. I know others speak of how much fun they have writing, but that ain't me.
A quick word about other readers. I held off. I didn't want to make someone else wade through my novel until that novel was my best effort. Other authors have a different approach and that's fine. You'll find yours; I just wanted to offer another perspective.
I've been prolix; I'll be succinct. Don't worry about where to begin. Pick a place and start there. Most of what you do, you will do better next time. But this isn't next time, it's this time.
Good luck, and you can rely on us for commiseration!