When you read the same piece for too long, it becomes difficult to edit. You know what’s coming and you stop reading from the page. But if you let it sit for a few days to a week, you can look through it again with fresh eyes.
Taking time away lets you see the writing for the first time again, making it easier to catch spelling and grammatical errors. And looking again with a refreshed brain allows you to think of different connections and clean things up.
Try to finish your piece a few days before your deadline so you can give yourself some time in between edits.