Proofreading
You probably need more proofreading than you realize. Your brain will correct things for you unconsciously as you read. If you go through the document a couple of times and don't see any errors, you should not assume that the errors aren't there.
You may want to make several passes through the document, looking for a different kind of error each time. Check your grammar. Check your punctuation. Make sure you haven't used an adjective to modify a verb ("He is quick. He moves quickly." Not, “He moves quick.”) Check your spelling, and give extra care to homonyms (words that sound the same but are spelled differently). "You paid to much" is an example of using the wrong homonym.
If you are going to proofread for someone else, you will have to learn standard proofreading marks. These are symbols used by professional editors to indicate when text should be omitted, inserted, capitalized, transposed, and so on.
Tips
Don't try to edit or proofread right after you finish writing the document. You need distance to get perspective. Leave it alone for a while, and come back with a fresh eye.
Consider asking someone else to look at the document. Things that seem perfectly clear to you may not be clear to other readers. Often, one person will catch errors that another person will miss.
Read the document out loud. This will help you notice clumsy phrasing or passages that don't quite make sense.
For meticulous proofreading, try reading the document backward. This gives you a new perspective and helps you to focus on one word at a time and one sentence at a time.
Watch for errors that you commonly make. Consider keeping a list of your own most common mistakes. This will help you keep from repeating them.