Think of the decline sit up as a more challenging version of the standard situp. For this movement, you will need a bench that has footholds. Because the bench is set at a decline, the footholds prevent you from sliding down the bench - which is pretty important!
Set your bench at a decline with the footholds up in the air and the opposite end of the bench towards the floor. Place your feet into the footholds, so that your feet are up in the air, your back is on the bench, and your head is towards the floor.
Using your abdominal muscles, perform a sit up. Be sure that you use your ab muscles to initiate our movement - not the hip flexors. To do this, keep a light grip on the footholds with your feet. If you find yourself pressing hard against the holds with your feet, chances are you are using more of your hip flexors than your abs.
Once you've sat up tall at the top and gently touched the footholds with your hands for good measure (this ensures that you're sitting all the way up), descend with control back to the starting position. This is one rep.
The decline sit up adds additional range of motion to the standard movement, so we recommend starting with a low volume of reps until you build up the strength. Once you become familiar with the movement, add more reps to your workout or perform them weighted by holding a dumbbell or kettlebell at your chest.