Editing is in many ways the most important part of the writing process. Good writing does not simply spring forth from head of the writer fully formed but requires effort and practice and though each person's process of editing is as different as their process of writing there are a few tips which can greatly help make your editing more fun, effective and efficient.
The first step in editing anything longer than a few paragraphs is to take a break. The longer you can afford to wait the more effective this will be. One of the most detrimental factors in authors editing themselves is that the words become their children. Kill your babies is good advice when it comes to writing, but it isn't easy advice, so stopping for a day or even an hour and doing something else will allow you to see the flaws. It will also make it far easier for you to notice confusing parts because you will no longer remember what you meant.
Once you have taken that break and are ready to begin editing I find that my best first step is to use the search feature(the most useful writing tool in any word processor besides spell check) along with a list of words that I know are problematic for me. Everyone should have their own list. Here are a few words to help you get started in making a list of bad words.
Very: This is almost always a hint that you need a better word. Almost any word that you can put very in front of can be replaced. Very big, can be replaced with enormous, very hot, scalding, very tired, exhausted.
Up and down: these words are used redundantly more often than any other words I know. He sat down, he stood up, he put down the knife on the table.
That: It could be useful, but you'd be surprised how often you can cut it and the sentence stays exactly the same. Then can also be problematic in the same way.
Begin searching your writing. You will find other examples of words you use too much. Searching for them before you begin editing can give you a good head start on the work of editing.
Edit for one thing at a time. This isn't to say that you won't fix a comma error if you see it, but the first time through you are likely editing for content so don't let yourself be drawn into proofreading. Most likely you'll be changing enough in the first edit that proofreading isn't going to be useful anyway. Edit for content, then edit for consistency, then edit for readability, and finally for grammar, spelling and polish. I like to think of editing as creating a sculptor. The first draft is a large block of marble, the second draft is cutting it into the correct form, then I began to shape the feature, and in the end I am going over it not with a chisel and hammer but fine sandpaper smoothing the rough edges. Most important though is to not attempt to edit too much while writing your first draft. Editing while writing is one of the chief causes of writers block.
It is also useful to know that if you are preparing a novel for submission you will most likely need a story summary. This is a point by point description of your novel and doing this while making a final pass over your work for typos and grammar can be helpful.
Once you have went over your work a couple times I then suggest checking your current draft against a first draft. This isn't strictly necessary, but it is often very encouraging. Editing often feels like less of an accomplishment than actually writing but as you see just how much better editing can make your work you will become far more excited to do it.
The last step is to know when to stop. Over editing is one of the most common procrastination techniques of many writers. This is of course a subjective decision but at the point when you are arguing with yourself over which word to use you are probably done with that part. Using the thesaurus can be useful but a few word changes aren't so important that you should delay submission to fret over them, there will always be something you can edit so it's important that you chose some point and simply claim completion, even if that point is completely arbitrary.
Editing becomes more fun and easier as you practice and for many of us we find that after a while it becomes more like writing than the actual first draft.