You should try to proofread a paper copy of the document instead of proofreading on a computer screen. It is easier to catch errors on paper than on a computer screen. Print the paper using a larger font (e.g. 14 point) so that you can see the punctuation marks more clearly.  Do not rely on your computer to catch all of your errors.You can use the spelling and grammar check on the computer before you proofread the document yourself. You can proofread the document on your computer screen before you print it out as well. For longer documents such as book-length manuscripts, you may want to ask to be provided with a paper copy or bill for the cost of printing Having a list of things to check for will help you stay organized. As a proofreader, you are checking for spelling errors, grammatical errors, punctuation errors, spacing, fonts, numbering, and margins. You can look for one error each time you go through the document or you may try to check for multiple errors at time.  For example, you may focus on punctuation as you read the text the first time and check spelling the next time you go through the text. If you are proofreading your own work, write down the errors that you usually make and pay special attention to them. You probably won't have time to use this strategy with a long work, such as a novel or dissertation, but reading backwards might be helpful for shorter pieces. Start at the bottom of the page and read the text from right to left. Reading the paper out of context will help you identify errors. Take your time and read each word.  Also read the paper out loud. This will force you the read more slowly and you may catch additional errors.  Focus on one sentence at a time. You can cover up the other sentences with a piece of paper or an index card to help you focus.  Use a highlighter or colored pen to mark the errors that you find. Proofread the text at least twice. You can alternate reading it forwards and backwards. Also, set aside time (e.g. 20 minutes, 60 minutes, 24 hours) between each proofreading session. Viewing the text with fresh eyes will make you a more accurate proofreader.  Always proofread in a quiet environment when you are not distracted. If there are a lot of errors, you will need to read the the text more. Consider having another person proofread the text as well. If you are in school, ask a teacher or tutor proofread the text. If you are in college, your school probably has a writing center that can help you.

Summary:
Print out the document if possible. Make a list of errors. Read the document backwards. Read the document multiple times if time allows.