Lock your feet under the leg brace of a declined bench and lie on your back.
Using a grip that is a little wider than shoulder width, dismount the barbell from the rack.
While holding the barbell, retract your shoulder blades (push them together) and straighten your wrists.
Execution
Keeping your elbows tucked in a little, inhale as you lower the barbell to your chest.
Exhale as you press the barbell back up to the starting position.
Repeat.
Comments and tips
Never use a thumbless grip (aka suicide grip)!
Do not lock your elbows out at the top of the press.
If lifting heavy, have a spotter ready.
Retracting your shoulder blades, keeping your elbows tucked in a little, and keeping your wrists straight help you to drive the barbell upward.
Keeping your elbows tucked in also reduces the pressure on your shoulders, which are prone to injury.
Compared with the standard barbell bench press, many lifters can lift more weight with the decline barbell bench press. The exercise also often feels easier on the shoulders.