Sit on the flat bench with your feet on the floor and glutes directly on the bench's crease. Then grip the dumbells and place the weights gently on top of your knees. Push your knees upwards and lean back to lie with a flat back on the bench. Keep your elbows tucked in as you lean back, once you are stable, flare them out to a 45 degree angle. Your wrists and forearms should be aligned with one another.
Start by pressing straight up, squeezing your chest as you extend your arms. Press until your arms are almost fully straightened, then slowly lower back down until the dumbbell gently touches your shoulder or what's most comfortable before that. Brace your core to keep your body stable throughout. Keep your shoulders back and down for the movement. Repeat for the desired amount of repetitions, then tuck your elbows in and gently rest the weights back onto your knees.
Avoid having your elbows flared out too wide where they are in line with the shoulders.