I agree with most of the good advice already on the thread, especially 1. Take your time, 2. Work in manageable chunks and 3. Be prepared to repeat the process.
Assuming you're beyond the point of grammar and spellcheck, the editing process means you need to be prepared to be decisive and brutal with your much-loved MS and those cherished characters.
One (admittedly not for the faint-hearted) approach I have found helpful is to set yourself a very tough word count target. This depends on how long your MS is right now, and you will need to do your research in your target niche, but let's say it's currently sitting at 100,000 words and you want to get it down to 70,000 words. That's a 30% reduction. Or every third word, if you like...!
I've cut a 150,000 word MS down to a 75,000 word book using this process. (Which I realise says a lot about that first draft!!! But that, in a nutshell, is the whole point of editing...)
You might not achieve your word count target, but the process will sure as hell force you to
edit. With that large a target you
have to look very hard at the whole construction of your MS.
These are just my thoughts - which may be entirely unsuited to your circumstances so as ever: only pick the advice that resonates for you.
Where to start? Well, to sort of borrow from the bard - first we kill all the adverbs. Search your MS for the term "ly". Delete them all. I said it was brutal. (Don't worry, you can add one or two back in later if you (real
ly) must!.
Then search for over-used words. I always search for "said" at an early stage. See how many of these you have, and think about how to improve your dialogue. "Said" is a common one, but you'll have your own "pet" words. One of mine is the word "glance". My characters are always glancing at everyone and everything. I've no idea why this horrible little word is stuck in my brain...it drives me nuts...