You already have some excellent advice to guide you as you help your daughter edit her work, but I add that you can still be a great editor even if you aren't the best writer. Editing is about much more than whether or not a sentence sounds good or grammatical errors.
Regardless of your daughter's age, the editing process is always basically the same, becoming more advanced and detailed as the writing does. Everyone has a different approach to editing and things like reading aloud and other suggestions here are all common practices.
When working with students on the editing process, I always give the same basic guidelines, which you can build out as you get better at it. First, I would suggest that you and your daughter come up with a set of questions to ask as you read her papers. Focusing your reading will make the process much smoother. Read for one or two questions at a time, then move on to the next one. Secondly, remember to look for ways to improve each part of the essay (each section or paragraph) and the essay as a whole.
When reading an essay, the three elements I encourage all of my students to look for are:
1) Coherence. Read for understanding. Questions you might ask as you read are:
-Does each paragraph make sense on its own?
-Do I understand the thesis?
-Do I know what I'm supposed to take away from this essay?
-Am I ever confused as I read?
2) Flow. As you read, pay attention to the order of things. Possible questions:
-Does my thesis lay out the order of my paragraphs?
-Is there a better order my paragraphs could be in?
-Does each paragraph have a clear topic sentence?
3) Grammar and spelling. This should be the LAST thing you read for. It's like the name written on a birthday cake. Get everything else finalized first, then pull the whole presentation with perfect spelling. If you're ever unsure of something, the Purdue OWL Grammar Guide is an excellent resource for most common questions and Noodle has plenty of helpful articles on the subject.