Proofreading is something I have been doing for years and for free but still, I never made any errors on my last draft. I do not rely on grammar or spelling checkers because they tend to have a limited dictionary hence why I always rely on my brain . I proofread one error at a time and I do not rush my work but I finish before the deadline. I circle every punctuation,  spelling mistakes ,  any error that may come across before I begin editing. My final draft is always perfect because I reread repeatedly to ensure the transitions between paragraphs are smooth and the grammar is good . Proofreading is also a learning process for me as I recognize and correct new errors along the way.