Aaron Mario of alpha m. demonstrates decline abs, which he stresses to do properly for effectiveness.
You need a decline bench and adjust the decline level. Lock feet in and then place hands across your chest. Go back until you feel a good strain and tension on your ab muscles. Do not go back or up all the way.
To make the exercise more difficult, grab a dumbbell or weight place. The higher you hold the weight, the more difficult the exercise due to the leverage point being Extend weight over your head for the most intense. Decline abs work the entire abdominal wall.