If you read my blog very often, you've read this before: the first step in editing is time. When your first draft is done, you get a feeling of relief. There. It's down on paper. It's done. However great that feeling may be, don't let it lead you to the mailbox. This is NOT the time to send your work to an editor, an agent, or even a friend. First it needs to sit for a while.
Walking away from a piece of writing is essential. I'm not sure how long it takes to be able to look at it objectively, and maybe the time needed varies with expertise and experience. I would say a first-time writer needs to give the work a relatively long time, say months, before going back to it and beginning to edit. If you're truly a writer, you will be able to see at that point where the holes are, where you've over-explained, and where you've fallen into whatever other traps you as an individual are prone to.
You'll use tricks when you edit, of course. Reading aloud is one of the best, especially with a passage that doesn't feel right. Reading the sentences in a paragraph backward separates them from each other and helps you see that each one is clear on its own.
How many times through a manuscript before it's ready to be seen by people in the business? Answers will vary. I edit in layers, each time through looking for one specific thing: sensory details, dialogue, character continuity, and, of course, mechanical errors. In between I like to have someone else read it, and at the last you should have one last reader check for typos and such. In other words, it's going to take a year or so AFTER I've written the last page.
I don't think you're ever done, but that's a good start. Let the professionals take it from there, and yes, they'll find things you didn't. It's the way these things go.