The harness can be applied lying down: be sure to attach the groinpiece to the rear buckles before placing the harness under the patient, to limit the number of maneuvers necessary.
if the person can “bridge” just have them lift up and slip the harness under them. Otherwise, place it under them the same way you would get a drawsheet under the patient: Roll them away from you, tuck it just more than halfway under, roll them back toward you and pull it through. Remember to position the bottom strap at the level of the greater trochanter (have them flex the hip to make it easier to find).
Feel for the buckles underneath the patient to make sure the harness is symmetrical underneath them. If not, do another series of roll-and-tuck maneuvers to reposition it.