Revision happens after the first draft. Revision is the act of re-seeing, of perfecting your language and ideas. Painters will go through a series of sketches to get to the final vision they want on the canvas. The same is true for writers. With each draft or revision, the writing moves closer to a final vision.
Many writers save grammatical and spelling corrections for the last draft.
I read aloud to myself and listen for the places where the language sounds thick. Those who seldom revise should do one more than they want to. Those who always revise may do one less. You know which you are. In a perfect world, we would revise until the essay was perfect.