So, you’ve written a huge, ugly monster of a first draft—something so distorted, scarred and ridden with plot holes that you're tempted to grab a shovel, bash the thing over the head, and bury it.
Don’t. You can save Franken-draft.
The first thing I suggest is outlining your book. Yes, outline it after you wrote it – even if you had an outline before you started writing the thing. You may have had a planned outline, but what did you actually put into the manuscript? A simple two column table in a Word document works for me. I use the left hand column to summarize the events in each chapter. The right hand column is for recording changes I need to make.
To help guide my revision choices, I also use a separate color-coded outline to analyze the way sub-plots are woven into the story. Again, I work chapter-by-chapter, boiling the events down to one or two sentences. In this example, I assigned purple to the central mystery, blue to a secondary mystery, and yellow to the romantic sub-plot. The color-highlighting helps me see where the various plot elements appear and make sure that they balance each other and that no sub-plot disappears for too long.
As for the actual revising, I prefer to make several successive passes through a manuscript rather than try to perfect the story in one revision. My second draft usually focuses on plot holes and character development. In my third draft, I refine the world-building (no matter if it’s historical, fantasy, science-fiction, or realistic). My fourth draft tackles voice, as well as the succinctness of the prose. This is where I get out my Grim Reaper robe, grab a virtual scythe, and get ruthless about word slashing.
Most of the time, you can remove just, even, and very without changing the meaning of your sentence. People can stand and sit instead of stand up and sit down. They nod and wave. (No need to say which body part is getting nodded or waved. We know.) Avoid phrases with multiple prepositions. In the back of becomes behind, and on the top of can often be reduced to on. I have a bad habit of identifying characters by their first and last names when one or the other will do. I also use multiple adjectives when a single precise one would be more effective.
By the time you've done all this, hopefully, Franken-draft has been civilized a little bit and is ready to bring out in society, or at least shared with beta readers.