What to cover
Scope (what's included and not), timeline, price, payment terms, number of revisions, what happens if the client cancels, and who owns the finished work.
Take a deposit
Asking for part of the fee up front (often 25–50%) is common and filters out clients who aren't serious. Start work when it arrives.
Handle changes
When a request falls outside the scope, reply with the extra cost and time before doing it. A friendly 'happy to, that's an add-on at $X' keeps projects profitable.