Editing will eventually turn to reading, which will eventually turn to skimming when it comes to a book you have written. If you edit when you are very tired, you will miss things that you normally wouldn't. Before it gets to the point where you can't focus anymore, take a break for as long as you feel you need. Sometimes it helps to take a few days off from editing; it helps you to see your work in a fresh new light, and you may find problems you didn't see before.
If you try to edit your work while you're still writing, you will not get very far in the writing process, and you may lose your creative train of thought while trying to correct typos. Finish the book, wait a couple of days and then go back for the editing process. When you must edit your own work, it's always a good idea to put some time between the writing and the editing; otherwise, you will probably miss many errors that you could have caught.
Do not try to edit the entire book in one sitting. The first thing you will need to do is read the entire book and delete things you don't like and add things you come up with. The second thing you will need to do is edit sentence structure and word choices. Then go back through the book and edit for clarity, spelling and grammar. Working in sections will help you to do a thorough job of editing in each area and remain focused on each editing task. Trying to fix everything at once will be overwhelming.
Authors tend to use the same words over and over again until they no longer have much meaning. For example, if you tend to repeatedly use the word "beautifully" to describe the actions of the characters, the reader will no longer get a sense of beauty from the actions. Look up words in a thesaurus and change frequently repeated words.
Use a dictionary for even the simplest words. Always remember that your reader may not have the same opinion about what a particular word means; always consider the possible connotations of your words. Make sure your language is precise enough to convey what you really want to say. For example, "gorge" and "graze" both refer to eating, but one means to stuff yourself and the other means to lightly pick at food.