1) Read through your work multiple times. It's easy to miss things. Read it again. Then again.
2) Look for different elements in each read-through. Does the story make sense? Does it flow well? Do your descriptions draw the reader in? Are your characters developed? Did you reveal enough at the right time? Is everything spelled correctly? Grammatically correct? There are so many things to keep track of when you edit, trying to catch everything at once can be overwhelming. Often times I'll do a read through to find technicalities, another for the flow of the story, and another for the quality of my writing style. Then I'll do more read-throughs. And then some more.
3) Take breaks between read-throughs. Get some distance from your work. If you're too close to it, you'll miss things. Try to pick it up as if it belongs to someone else.
4) Try out different formats. This has been a particularly helpful method for me. I read it the way it appears. Then I zoom in on the document and read it again. Not only is this easier on the eyes, but it helps you catch small errors like missing letters or words that you've missed because your mind added them in for you when the text was tiny. Also, read through your book when it's on your kindle for the first time. Read through it again when you see it in a physical book. You'll catch new things every time.
5) Read your book out loud. This is another favorite of mine. It's the perfect way to see how well your story flows, plus if you read slowly in large print, you'll catch more of those tiny errors. It's a great editing tool, and you can practice speaking as well. You don't have to read out loud to anyone. Personally I enjoy reading alone or to my cat. Sometimes I'll read to my boyfriend, but only when he's asleep. You get the picture.
These are just the editing tips that I've found most helpful. There are tons more, but I'd encourage you to get to work instead of dinking around on the internet looking for them. Happy editing, and good luck!