Editing your piece of writing is not about making sure the grammar or spelling is correct (save that for when you proofead the draft that for you is final); it's about making sure that the content is at its full potential.
In your first big edit of a piece of work, you want to make sure that you know -- and are conveying -- the main point, as in why you're writing this piece. It may take a few revisions to fully unearth the main concept. (Remember: The secret to good writing is revision!)
To help yourself with the big things, consider some of these questions as you go over your piece. It's always easier to write more than you need and cut it later than it is to scramble and add more at the last minute. (Hint: On your first big edits do NOT worry about the little stuff; read it through, then ask the questions.)