Unless you hire your own proofreader to look over your essays for errors, the burden of proofreading will be on you.
After you’re done with writing and editing your essay, you’ll have to proofread it. Proofreading is about sentence-level stuff: correcting misspellings, run-ons, and other problems with grammar and mechanics. If you’re using word-processing software, pay special attention to words underlined in red and green squiggles. Not that Microsoft Word always gets it right—so look these over with a discerning eye. You know what, Microsoft Word? Maybe you’re a sentence fragment that you should consider revising. Did you ever think about that? Huh?
Before you start proofreading, you’ll first need to have a good, solid essay draft that expresses intelligent ideas and expresses them well. If you proofread before you have a strong draft ready, you might end up working on something that ends up being cut or dramatically revised—which can be a huge waste of time.