Put it aside for a few days, or longer, do other things, try not to think about it. Then sit down and read it (printouts are best I find, but that’s just me) as if you’ve never seen it before. Start at the beginning. Scribble on the manuscript as you go if you see anything you want to change. And often, when you get to the end you’ll be both enthusiastic about it and know what the next few words are. And you do it all one word at a time.