How does a physical therapist help you use a prosthesis?

Multiple ways. A good PT will help the person by teaching them how to put on the prosthesis appropriately, learning to identify and troubleshoot prosthesis fit problems and help re-learn walking with the device. Their role is crucial in Amputee Rehabilitation.
Balance. You will learn to walk and balance with a prosthesis. Since the prosthesis is an extension of the stump it takes getting used to.