Proofreading means examining your text carefully to find and correct typographical errors and mistakes in grammar, style, and spelling. Here are some tips.
Be sure you've revised the larger aspects of your text. Don't make corrections at the sentence and word level if you still need to work on the focus, organization, and development of the whole paper, of sections, or of paragraphs.
Set your text aside for a while (15 minutes, a day, a week) between writing and proofing. Some distance from the text will help you see mistakes more easily.
Eliminate unnecessary words before looking for mistakes. See the writing center handout how to write clear, concise, direct sentences.
Know what to look for. From the comments of your professors or a writing center instructor on past papers, make a list of mistakes you need to watch for.