Here are the some instructions I received from one of my clients. I find them to be very simple and very good in terms of explaining the role and boundaries of editing, which is not always an easy task.
It is always easier to translate than to edit someone else’s writing. Yet we cannot impose our individual writing style on another individual. Therefore, a good editing job focuses on terminology, grammar, syntax and consistency of expression.
My clients give my comments to the translator, who accepts some changes and rejects others.
On top of editing, you can add proofreading, i.e. looking for typos, spelling mistakes etc.
Hope this helps