Ride the horse in a menage or enclosed space until you get used to it and handle it a lot in stable and yard - take it out on a quiet rides with a sensible horse/rider who knows the area and will keep you out of potentially tricky situations - obviously wear helmet and preferably a back protector. Get the loan agreement down on paper to include period of loan, who pays for what and when together with information should the owner want it back or you want to return it for any reason - sort out insurance for vets bills and liability as third party damage caused by the horse can be very expensive. Loans work well if done legally and responsibly but can go horribly wrong if not.