The best way to modify the sit up is to do a roll up with the knees bent.  The feet are firmly planted on the ground. With the arms straight up in line with the soulder line, exhale the breath and roll the body up one vertebrae at a time without lifting the feet off the ground. Start with the feet further away from the buttocks until the abdominals become stronger.

I agree, you should try to attend wood class at Crossings on Thursday evenings at 6:15pm or Saturdays 12noon at Uptown. The wood class will teach you the correct method to strengthen the core abdominals and how to engage these muscles during your yoga class.