Hi Marieta! Keep in mind that sit ups are an advanced exercise (as is the entire August rotation;-) ) so if at any time you find them to be too stressful to your body in any way, do partial sit ups (bringing your body up only part way).
Sit Up Form and Technique:
Start: Lie on your back, knees bent, feet flat on the floor, and hands either behind the head to support the neck or across the chest in front (your choice). To make the exercise a little easier, anchor your toes under a heavy object to give you support on the way up.
Execution: Before you even leave the floor, contract your core. Then follow through by lifting your body up off the floor in a rolling up fashion to a full seated position. Without resting at the top, lower yourself down (with full control) to the starting position.