Hold your phone sideways like a steering wheel. Tilt to steer, hold GAS to move, and tap the gear button to switch between R and D.
Stages
Your rig
Defaults: F-350 crew cab, 8 ft bed (176 in wheelbase), 35 ft fifth wheel with 27 ft kingpin to axle center. Check your trailer's spec sheet and adjust.
Parked!
Turn your phone sideways Hold it like a steering wheel. If the screen won't rotate, turn off Portrait Orientation Lock in Control Center.