Ive found the best beginner position for strap on is with the receiver laying face down on a firm surface with his knees on the floor. This allows you to relax your lower body muscles completely. If you are in K9 position you will tend to tense up your muscles to counteract the thrusts of the giver.
There is no script though. You will have to see what your provider does. Hopefully she is experienced. Ask her to start
slow, make sure YOU verbalize and have lots of lube on hand. And have FUN, you will love it