You’ve finished the book, and you have the overwhelming urge, upon typing the last words, to immediately print the thing and send it out in the mail. (At least I do. By the time I finish my books, they feel like houseguests who have way overstayed their welcome. I want them out the door yesterday.)
Take ten slow breaths, say your mantra, hit your thumb with a hammer—do whatever you have to do to put the brakes on. Don’t send the book out yet.
You have things to do to it before your editor ever sees it. Go ahead and print out a draft copy. Get out a notebook and a pen. Start reading through from the beginning. If you find typos, fix them of course, but you’re looking for more than typos. What you’re doing now is a continuity check. Make sure that your days and hours track—that if a full moon hung high in the sky at the opening of the scene (meaning the time was right around midnight) you don’t have the grandfather clock tolling six. Or that your hero’s eyes don’t go from brown to blue halfway through the book. Or that you haven’t left a scene with little markers in it that you were going to go back and fix when you figured out what happened there, without ever going back to fix them. (I’ve found whole sections like this in books I’ve thought were finished, and I’ve been ever so grateful that I took the time to go through the manuscript before I sent it out.)
Also look for clumsiness in the writing itself and places where you used almost, but not quite, the word you intended. Check for places where you spelled a character’s name in different ways, and so on. By going over the manuscript as carefully as you can after completion of the first draft, you’ll make sure that what your editor reads is what you sent, and not what you think you sent. There can be a world of difference between the two.