Proofreading is the very last thing you should do before you turn in your paper. If you do these before you've finished revising and editing, you could introduce new mistakes during the course of revising and editing.
Proofreading is all about the details. Details are inherently minor, until there's a mistake and then they can be incredibly distracting. A few details that are often overlooked include:
It might seem counterintuitive, but one of the last steps in proofreading is to zoom out (or use the print preview function) to look at each whole page of your paper all at once. It is a quick an easy way to find anything that shouldn't be there or is missing.
Things to look for when zoomed out: