Be sure first that you have a great amount of knowledge or experiences with punctuations and grammars before proofreading. But if you know what is right, then go on, but if not, then proofreading won't be a use at all.
1. Keep yourself from getting distracted, focus. Look for a quiet place to proofread where it is far from all the disturbances to help you focus.
2. Don't read the piece silently. Hearing can be sometimes the best judge with grammatical errors and sentence constructions.
3. Try to be a little soft on yourself while on the job, taking little breaks won't hurt, but instead will help you calm and set your mind back to regain your focus, for sometimes doing something too long makes you lose your focus.
4. Put in mind that this is not just about grammars and spellings. Styles can also be a great factor, and having inconsistency in style can be a great downfall. Make sure that your style is consistent, like your headings having all the same font style and size. Do this line by line, and use objects like rulers to guide your eyes to avoid skipping of lines.
5. Word by word it is, you need to practice to read sentences or paragraphs word by word with great focus. Normally, when people read they tend to just slide through the lines, which can make us not notice some small errors. Generally, most of us rely on our not so accurate peripheral vision for most of us can only focus on six letters in one stare. Effort is needed, just for a short period of time, make everything around you go slow and proofread slowly word by word, letter by letter.
6. Take a little break, and don't proofread after writing immediately.