Sitting down with your body upright (no slouching allowed) with hands gripping the bar in an under hand position, just wider than your shoulders and with arms straight.
Keeping the body still, start the move by pulling your shoulders back and then down. Imagine trying to pull your elbows down. At the bottom, really squeeze the shoulder blades together and hold for a moment or too.
As you let the bar move back upwards do it slowly and deliberately and don’t stop until your arms are fully stretched.
Make sure you don’t round your shoulders at any point and in this grip, don’t be tempted to lean back.