Every manuscript (MS) needs editing, period, full-stop. And I don't mean the "get the commas and semicolons in the right spot" editing. This is the editing that looks at the story, the characters, and the thrust and parry of the interchange between the characters and the story. This is also where you need to take out all the parts that people don't want to read anyway (thank you Elmore Leonard). Even the best writer needs to step back and think about what to leave in and take out of the manuscript. I'm reminded of the whole cannibal episode in Dashiell Hammett's The Thin Man, why it's in there is still a mystery to me.
Here are some helpful ideas while attacking that editing job.
Compile Your Notes
After assembling input from readers and hopefully your story editor, organize their comments and observations in order of magnitude from most impactful to least as regards the story. Use them as a guide as you begin the rewrite. Keep this reminder posted on the wall next to your computer as you rewrite.
Color Key the Text.
Example: In my latest thriller I have three primary characters, the protagonist (the good guy), the antagonist (the bad guy), and a married couple that are fundamentally one character aligned with the good guy. To understand where these players interact (or how much space they take in the manuscript) I color key (red, green, blue, etc.) the chapters where they are the dominant player or players. Then in Word reduce the page view to 10%, the flow of their on-stage time becomes apparent, this will give you a quick look at where rebalancing may be needed. There are variations on this regarding scenes and actions, develop your own story coloring, it can really help.
Everyone says that you should take a break from the MS for at least six weeks; I used to think this was silly. I don't anymore. After this vacation and armed with the comments from your readers and editor it is surprising how "new" the story seems and the flaws obvious. It is also easier to cut out the extraneous. If it doesn't push the story forward it is a good bet it's not needed.
"Ideas for the Future" Folder
I have already written one complete thriller from the inspiration of an excised chapter of an edited manuscript. The removed chapter was no longer was relevant to the first story but it was a good piece of work and became the inspiration for a whole new book. I have another removed chapter that begins a new book just waiting to be written. The adage "never throw anything away" is very true. Open that folder right now.
Jump in and push yourself as fast as possible with the first major rewrite. At this stage continuity becomes important as well as character development. Be mindful of what is going on in the story, connections, as well as shading for future events, and potential traps you are laying for your readers. If you can't fix it right then make a note in your manuscript notebook and make the revision later.
When you think it's done, it is not. Your last rewrite should be slow and grinding, question almost everything. Do the coloring of characters again, is there better balance, do they grow and change? Look to the text and the language, does it work? Is it gold or dross?
I have heard that some writers spend more time editing then writing, maybe. I can see that happening. But remember it is extremely hard to write a good story and perfection is, sadly, impossible. But damn good is all right in my book.