Think of this as the garage lifter's lateral raise machine:
Start by orienting your body slightly inward, toward the barbell, with the business end aiming at the hip joint of your outside leg. The constrained path of the bar provides some guidance through the motion.
As a bonus, the barbell's rotating sleeve allows you to begin in a position of shoulder internal rotation and end in a position of shoulder external rotation.
Rehab pros might note the similarity between this exercise and the PNF D1 pattern, which incorporates tri-planar movement of flexion, abduction, and external rotation. Keep a slight forward lean throughout the movement.