When to Proofread

Proofreading is the final stage of production in the writing process. This is the stage where final mechanical corrections, such as grammar and punctuation, can be focused upon most effectively. After higher-order concerns, such as development of ideas, organization, and logic, have been addressed and a final draft is ready, you can begin analyzing errors in your writing.
If you proofread while you are still in the stages of developing your paper, you can waste time since individual sentences may be modified, added, or deleted.

Proofreading occurs after revision of the rough draft and subsequent drafts. Even if you feel sure that you have no errors, never turn in a paper that you have not proofread since the combination of human and computer error makes it unlikely that your paper will be perfect without you reviewing it first.

After proofreading the first time, leave your writing alone and look at it the next day. Sometimes readers numb themselves to what they are looking for, so having a break can give you a fresh mindset.

Also See:

Proofread Revise Polish

Why Proofread

proofreading marks / symbols

Proofreading Tips