When I proofread an article or a story; I follow some very simple rules - Is the message being delivered; if not where do I get lost, confused, or even worse bored. Once that is done, I will look for grammatical issues.
Once I have identified those areas, I relay them back to the writer with suggestions on how to correct these problems. If there are technical or factual errors, I will point the writer to my sources.
I hope this explains my process on proofreading. If you have any questions please contact me. Thank you.