I've written about self-editing before and I'm positive I'll do it again. I'll probably even repeat myself. Self-editing is a crucial step in the writing process and learning to do it well will save you--and your editors and beta-readers--a lot of headaches.
1. Read it out loud.
Listen to the flow of sentences, your word choices, and your dialogue. Reading aloud also helps you find those missing words you thought you wrote but totally didn't.
2. Have it read to you.
Back in the days before on-screen readers, I used to have my youngest sister read my fanfiction to me. Now, you can just save your file as a PDF and have Adobe Acrobat read it to you.
3. Read it backwards.
This step is useful for spelling errors more than anything else. Start at the end of your document and read backwards, one word at a time. It might also help you see if you fall into the trap of using the same vague words more toward the end of the text, when perhaps you're tired and frustrated and not as fresh and excited as you were at the beginning.
4. Find and replace "problem" words.
First you have to identify your problem words, which I would define as words you use excessively. I like to do a quick find-and-replace and substitute an all-caps version of the word for the word itself, that way my eyes catch on it and when I'm looking at the document as a whole, I can see the placement of the words. After that, I go in, look at, and edit each instance of each word.
5. Find and replace pronouns, and, then, as, like, was, were.
I like to get a sense of how often I use s/he, his/her/its, him/her/it, and the rest. I copy #4 for this tip.
6. Ignore it.
Sometimes, ignoring it is all you can do. The longer the work, the longer your break should be. When you come back to it, you'll see it with fresh eyes and read it with new understanding.
7. Save it somewhere else and read it from there.
If you write on your laptop, read it on your Kindle or iPad. Print it out and read it on the back porch with a glass of wine. Change your surroundings to jar you into seeing the piece differently.
8. Change font, size, color.
Simple tweaks like using a font you don't normally like in a larger or smaller size or different color can change the way you see the manuscript.
9. Read it as a reader, not as a writer.
Turn off your inner editor and just start reading. Don't do any editing until after you've consumed the work as a reader.