"Rewriting is the essence of writing well: it's where the game is won or lost." —William Zinsser, On Writing Well
What does it really mean to revise, and why is a it a separate step from editing? Look at the parts of the word revise: The prefix re- means again or anew, and –vise comes from the same root as vision—i.e., to see. Thus revising is "re-seeing" your paper in a new way. That is why revising here refers to improving the global structure and content of your paper, its organization and ideas, not grammar, spelling, and punctuation. That comes last.
Logically, we also revise before we edit because revising will most certainly mean adding and deleting and rewriting sentences and often entire paragraphs. And there is no sense in editing text that you are going to cut or editing and then adding material and having to edit again.
Revising is also part of the learning and discovery process discussed earlier. As you reread your paper, you may see weaknesses in your argument that need strengthening, and you may thus have to do a little more thinking and research. Or you may have to restructure your paper somewhat to make the argument more logical. Keep your mind open to the possibility of learning in this stage.
You also do not have to revise alone. Not only will your professor look at your paper, but your classmates, colleagues and friends are great resources for getting a fresh, outside look at your work. Be sure to ask them what does work—we all need encouragement, especially as writers—as well as what doesn’t.
Continue to step-by-step instructions for revising.