Never had a contract with my pet sitter.
I would write something up like wne you come (e.g. every day at approximate times 8 am and 6 pm), what you do each time, replace water in buckets after rinsing buckets out, prepare feed (with medications), blanket, hay, turn-out, put in stall for evenings, bring in mail, feed dog/cat, etc…
Detail cost per feeding (in case you end up with 1/2 a day), what you expect from owner, payment (do you expect payment in full before sitting, 1/2 on first day and rest with 24 hours of returning home - be certain to provide mailing information for payment or set a time when you can come pick up the check?), house / barn keys, etc.
What to do in case of fire, how to contact owner, vet information (and when to call them, when not to call them)…
You want to CYA and manage expectations for both parties.
Also provide a phone number for owner to reach you and specify the MAX number of times per day and minutes per day they can call you (if you give them a cell phone number)… may sound like overkill but a friend had to block one ADD lady on the phone as it was costing her a fortune as she had a limited minutes plan.