You've done it. You've just finished the first draft of your masterpiece, and now... Now it's time for the long slog that is the editing process. How should you go about it? How much editing should you do? Should you hire an editor? Just like my last post on writing, everyone is going to have their own process, but I'll outline my process here.
Step 1: Reread your manuscript. This should go without saying, but after you're done, you need to reread your manuscript all the way through and with a fine-toothed comb. Fix grammar and spelling errors as you go. Check yourself for continuity errors carefully. Unless you're a machine, you will have definitely made continuity errors.
Try to check tone, believe-ability and style, but this will be a lot harder. It is really really difficult to try and be objective when editing your own work for style and tone because you're not coming to the table with a clean slate. Your reading of your own work is always tainted. You wrote it. You have a mental image of the scenes in your head. The reader will not. That's why I'm a big proponent of getting other people involved in the process. After you're done giving your manuscript a once-through, it's up to you if you want to go through it again before moving on to step 2.
Step 2: Beta readers. Get some trusted friends (who don't mind reading a first draft manuscript) and send them copies of your manuscript to read and give critiques on. Try to find people who are avid readers, maybe with an English background, and people who enjoy your genre. But mostly, get people who you think will be honest with you. Deep down, we all think we're great writers. But you're first draft--how should I say this--will not be very polished. Despite your best efforts, you will have missed things. Some big. Some small. It may hurt to hear, but if you want your work to be the best that it can be, you need to pick beta readers that will have the guts to be honest with you. I usually give my beta readers a month or so before I start bugging them :)
Personal note, I have one additional step between Step 2 and Step 1 that may not apply to a lot of writers: my wife (who I call my Alpha reader). She's a huge sci-fi nerd like me but also has a good head for what's realistic and what's not. Before I send it out to my beta readers, I do a full edit with her, which takes care of a lot of my bigger manuscript issues. But if you don't have a partner willing to cut through your unedited writing, then skip this part and go straight to beta readers.
Step 3: Full reread. By this point, I've gotten input from my wife and four or so beta readers and changed large swathes of the book, which is just begging for more continuity errors. So, do a final read through again of your book to try and scoop any of those up.
Step 4: Professional editor. I am a big proponent of using a professional editor. I don't care if one of your beta readers has an English degree, editing a novel is very different work. Having a professional editor can be the difference between a polished piece of work that gets solid professional reviews and one that's rough around the edges and goes in the "eh" pile. Professional editors are all over the place on the internet and can be quite expensive (on the order of a few cents per word, which adds up fast). How do you find one that's right for you? They will usually have a list of services and recent clients on their websites. Don't be afraid to ask for examples of their edited work or even sample-edit a couple of pages of your manuscript. If you really want to compare editors, give different ones the same couple of pages to edit and compare them. This is a relationship that could last for a long time, so be choosy!
Your editor will usually do several passes of the manuscript before handing it back to you with tons of edits that you can then slowly work through together.
Step 5: Proofread. Copy editing is not the same thing as proofreading. Proofreading is the very last thing that you do before you publish where you go through very carefully for small spelling errors or grammatical errors and don't pay attention to anything else (all larger things like "it would have more oomph if person A said this to person B instead of that" will have already been hammered out by this point). Some people skip the proofreading part, and others don't. Your call. Your professional editor may offer this as part of a package or you may get a separate professional to do this.
However you do it, don't get too excited. Editing can take months and months and be a very frustrating process because you're dealing with other people's timetables, so be patient! The tendency is to feel like it's taking too long and just throw your hands up and say, "eh, it's good enough," and submit it for publishing before it's ready. Don't do that! Make your manuscript as polished as it can be before you submit it, because you'll regret it later if you go off half-cocked. And just like in my previous post, be open to new ideas and go with the flow as best as you can :) It can be hard inviting others to critique your work, but in the end, it's well worth the wait.