Position your legs securely in the padding of the decline bench and lay back.
Grip the barbell as you would for a close grip bench press.
Lower the barbell to where your elbows are roughly 90-degrees.
The bar should be at or just above your chest.
While keeping your arms in that fixed position, move the weight behind your head in a semicircular motion until the bar is behind your head, parallel with the floor.
You should feel a strong stretch in the lat.
Don't beyond your range-of-motion. Your mobility/flexibility will improve through performing this exercise consistently over time.
Return to the starting position by contracting the lats and performing the opposite semicircular motion.
*Tips*
The arms do not initiate the movement. They are levers for your lats.