If I am being asked to do a full tidy up of a work I start with the proofreading and scan over each paragraph and focus more on finding any grammar, vocabulary, or spelling errors. Once I've finished with the proofreading I move on to the actual editing where I go back and really read what I've been given and make sure fine points have been met. Is the target audience being met? Is there a good flow through the entire work? Are there any facts (historical, scientific, statistical) stated that need to be verified? If I'm working with fiction I look for consistency in plot, character development and interaction, writing style and voice. If I find plot holes or story lines that end abruptly with no explanation I try to work out and advise a bridge to cut through that and make it clearer for the reader. When I've finished with editing, I read it all again as a "new reader" and just kind of enjoy the story, or learn whatever information is being stated and see if it is clear and can easily be understood and enjoyed.