Ask HN: Consulting work on the weekends -- what should I know to protect myself?

An old coworker has asked for help in an area I have an expertise in. I've told them I can only work on the weekends and evenings, so that it does not interfere with my current job. We'll be signing a contract -- what should I look out for? How do I properly measure time worked? How do I handle emails (do I time myself when I read and respond to emails to bill?).

Any advice is helpful, thank you.