Proofreading involves making light or stylistic changes to a text, ensuring a consistent tone and register, and polishing a text into a final draft according to an already-specified standard of quality, which usually depends on the project in question and the client’s goals. I follow best practices for clear, concise, coherent, and compelling content.
I also use the guidelines and criteria of English-language style and usage manuals, such as Garner’s Modern English Usage (5th ed.). Bryan Garner’s masterwork is my eternal friend.