Have you ever been to a posh dinner and been presented with a vast array of silverware spreading in ranks either side of your plate? Editing is like dealing with all those forks and spoons without getting it wrong and spilling soup down your front, or using the butter-knife to eat your peas. The simple answer is to start from the outside and work your way in.
The first thing to do is put the book away for as long as you can manage. The longer you leave it, the more distance you have. The more distance you have, the more you read like a disinterested reader, and the more you're able to spot problems. There's what you think you wrote and there's what you actually wrote, and if you're too close you can't see if there's a gap.
When I did this on my first novel the gap was about four months, mainly because I was incensed that the world hadn't realised what a startling work of genius had just landed on their doorstep and turned it down. Cue metaphorical flouncing out of the room and mega sulks from me. When I did finally go back I was ready to concede that the world might have a point.
At our imaginary dinner party this would be like the soup course. Soup is a lovely liquid mass, contained within the bowl, but can flow anywhere. Look at the cutlery and choose the spoon furthest away from the plate - you're working from the outside inwards, remember.
The reason I say go outside inwards is it makes no difference how beautiful any individual sentence is, if the whole thing is wrong, if the story telling doesn't work, if there are problems with structure, then no one is ever going to read that perfect sentence. So, the story, the structure, the shape has to be right before you start fussing over adjectives and verbs.
At this stage I like to put the story down on index cards, one card per scene. On the card you write the setting, characters present, the purposes of the scene and the main action. This is one I wrote for an early draft of Another Woman's Husband.
Setting: Don't actually know! next page, B's house somewhere. Also, not dated.
Becca and Lily. Becca dreaming of Paul, Lily wanting to go out late clubbing. Frank rings, wants Becca to go round and help. NB Frank last mentioned/seen when? Pages ago.
And a couple from Kissing Mr Wrong...
Setting: Lorna's place. Dinner party. L's invited Marcus for Alex. Other people there NB should have been mentioned in opening scene. Skiing trip mentioned - Alex will need to find the money. Lorna offers her job in the gallery.
Setting: ????? Alex and Lorna. Alex talks about a) career, she's gone adrift b) Marcus as perfect man c) what to do about photograph. Lorna a) tells her M's going to Glasgow b) suggests Gus as possible re photograph
Obviously, as I was writing out my index cards I realised there were some problems which would need to be addressed should the scenes remain in the next draft and made notes accordingly. But that's for a future stage. Right now I'm checking that it's clear what the purposes are for each scene and how they move the story on.
When I've gone through the whole of the novel I've got a stack of index cards. I lay these out on the bed (I work a lot in bed). This is the easiest way to 'see' the novel as a whole. I'm looking for various things, all concerned with structure -
* is the 'shape' of the novel right, with exciting stuff happening throughout
* is there a good balance between active and reflective scenes (ie pace)
* do scenes flow ie have I set actions up
* are there any obvious holes - a character goes missing for a while, a plot strand is unresolved
* is the timing right? eg if someone becomes pregnant in January, do they have the baby in the autumn? At this stage I work out exactly when each scene takes place and note any bank holidays or other events that may affect the story.
I move scenes around, I add them, I take them away, I combine them. Anything. It's a fluid process (it's soup!). When I'm happy with the shape of it, it's on to the next stage.