I've been editing for quite some time, and one of the biggest challenges I have is helping writers edit their manuscripts. For most authors, their books are their babies, and they simply can't bear to change a single thing. The most successful editing jobs I've ever had are those where the writers come in prepared to rip apart and dismantle their entire manuscript. Of course, this isn't easy, and I fully understand. It can also be even more challenging to do this yourself, because we often fail to realize which parts of our work are weak and need improvement.
Fortunately, there are a number of techniques I've come across to help you edit your manuscript in a way that is as efficient as possible, meaning that you produce work of the best quality with the least amount of time and energy wasted.
1) Read it aloud. I cannot stress how important this is. It will not only force you to evaluate each word individually, instead of skipping over bits of phrases and missing mistakes, but it will also allow you to examine the flow and the wordiness of a sentence. If you have an ample amount of time, I would suggest spending the first part of your editing session simply reading. For example, if you have a novel spanning thirty chapters, take the first five pages and decide you will edit these during this session. Start by reading them aloud, and simply put an asterisk (*) beside any parts you find awkward, confusing, wordy or otherwise incorrect.
2) Grab a thesaurus. As you read your story, you might find that some words are repeated far too many times, something most of us don't catch in the writing process. This is your chance to get your thesaurus and find a variety of words to replace your repetitions.
3) Punctuation. Correcting punctuation mistakes can be difficult, because oftentimes you might not know all of the rules. It's therefore very important to pick up some sort of no-nonsense grammar and punctuation book that will plainly explain the rules. This book should always be kept near your writing space for frequent reference. Alternatively, there are many websites that offer the basic rules. Just make sure that the site is trustworthy, or else you might be reading false information.
When correcting punctuation, you should evaluate each punctuation mark and try to see if there isn't a better one. Reading aloud is very important for this process, as you can see where there are run-on sentences that should be cut down, or find out where a comma should be placed to better the flow of the prose.
Once you've done all this, make sure to run a spellchecker to make sure that everything is in place. If you come across something you missed, try to realize what mistake you made. Is it a rule you weren't aware of, or merely something you weren't able to catch?
4) Consistency. Try to create a checklist of facts you mention during your manuscript. For example, if in the first paragraph you say that a girl has blue eyes, note this. If, in a later chapter, you change her eye colour to green, you can observe your list and realize that you're lacking consistency in your facts.
Also keep your punctuation consistent, as well as your verb tenses. If you plan on writing all your dialogue in single quotations, don't suddenly flip over to double quotations halfway through the manuscript. Another very common mistake that writers make is starting off their prose in the present or past tense, and then slipping into another tense at various intervals. It's essential to keep your verb tenses consistent, so make sure to check for that.
5) A second pair of eyes. If possible, get an honest and sincere friend to go over your manuscript. Not only might they know grammar rules you're unaware of, but they might be able to pick up problems with your characters or plot. While your plot might seem fascinating and fast-paced to you, for your friend, it might drag on and be boring. It's important to know this so that you can alter it and make it more interesting.
If this isn't possible, the best thing to do is let your manuscript sit for as long as possible. Anywhere from a week to a month is best. Afterwards, read through it purely to observe the plot and characters, and be honest with yourself. Could you make it more interesting? Are these the best characters you can create? Do this with the idea that you already know there's something wrong, you just need to figure it out. Don't have a mindset that everything's perfect, so you'll be very surprised if you can find anything wrong with your book. This way, you'll never pick up on any mistakes that should be edited.
6) Try to edit when you're feeling considerably energized and motivated. Otherwise, you'll skim through the text without giving your full attention, and you might miss mistakes you would otherwise catch with ease. Writing is an art, and you owe it to yourself to take the time to carefully edit your manuscript, in order to produce work of the highest quality. Don't allow laziness or an incapability of critiquing yourself to get in the way. At the end of the day, you'll thank yourself for it.