Three options here.
Using the hanging stand next to the Back Extension bench
Press your back against the back rest, grip the hand pieces, and rest your forearms along the arm rests. Take your feet off the foot rests and allow the legs to hang down. Keep knees and ankles together, bend at hip and knee and bring your legs up towards your chest, hold for a second, and slowly lower. Aim to get up to 5-10 lifts but if your abs complain, take your time.
Bending the knees straight up will work rectus abdominus. To work the obliques, keep knees and ankles together and lift sideways to bring both knees to under the left or right hand grip. Aim for 10 each side again - as you get better aim for 15.
When 15/20 is easy, increase the length of the lever (your legs) to make it harder. Keep knees and ankles together but instead of lifting your knees to your chest keep your legs straight and S-L-O-W-L-Y lift your feet forwards. Lower them equally slowly - it's really hard work! When your abs say they've had enough, take notice and stop - abdominal muscle spasm is not nice.
Doing straight sit ups using the sit-up bench
Increased angle = increased difficulty. Set the bench at the right angle for you. Higher = harder, so start at the bottom.
Put your feet under the foot bar and bend your knees so that your feet are flat and your back feels comfortably supported - if your knees aren't bent enough it doesn't feel comfortable.
Put your hands across your chest or have your fingertips touching the sides of your head
Slowly raise your head, neck and upper back off the bench
Keep looking at the point where the ceiling and wall join - DON'T look forwards at the wall! Keep the movement steady and don't jerk your head/chin/neck forwards - you don't need to lift all the way to a sitting position to work your abs very effectively. Keep the movement controlled and lower your head. To make things harder, don't quite touch down before you lift again.
To work harder you can do pulsed sit-ups. Lift your head and shoulders and then move a small distance backwards and forwards without lying down again - aim for ten pulses and then lie back.
Sit-ups should be silent! - don't tense yourself up, lunge forwards and grunt!!
Using the Swiss Ball
Sit down, walk your feet forwards and lie back at the same time until you're effectively lying with the ball in the middle of your back. Cross your hands over your chest and lift your shoulders off the ball - you don't need to lift high to make the abs work. Aim for 10 lifts then increase to 20. This is a good way of working the abs if you've got lower back niggles (although make sure you don't increase the pain)