Simple, not easy. Urdhva Hastasana in Tadasana bringing the arms up from the sides.
Start with your arms extended out to the sides with the hands at the same level as the shoulders
Drop the inner shoulders, upper trapezius, down
Retract the bottom of the outer shoulder blades in
Straighten the arm completely engaging the triceps
Maintain these actions as you turn the palms towards the ceiling
Slowly raise your arms up without losing the actions of the shoulderblades