I was looking for some sketchbook paper and found pages and pages of notes on Editorial. I remember when I was offered the position I just didn't feel ready or like I knew what I was doing. So I read several books on editorial and took some notes. Here are some good ones:

Do no harm. Never alter an authors individual style or change the content or meaning behind their writing. Don't adjust an authors language because it's not to your taste.

Respect, Clarity, Conciseness

English is flexible.

Take pride in your work, remember that every book you edit is a reflection of your professional skill.

Keep an eye out for "pet words". Words or phrases overused by authors.

Coaching vs. Modeling. Coaching is giving suggestions and modeling is showing what you're looking for.

Another note I've learned is to be specific about what is not effective and suggest to the author what ways they can be more effective. Nothing is more frustrating than being told what's not working and then being left to my own devices to fix instead of troubleshooting together.

Also ask questions!
"Is this your intent?"
"What are you trying to get the reader to walk away with?"
"How are you feeling about this?"
"Are we on the right track?"

Conversations can reveal more than just giving directions.

Anyway, I'm very fortunate to be able to work with my few editorial clients while working on my own art. Being able to edit and be edited at the same time is good for me creatively, I think. And I hope makes me a better editor! I love editing!