1. What is the safe way to do sit ups?
Sit-ups are still a popular exercise for either helping back pain or simply toning up the abdominals. Current thinking is that sit-ups may be an acceptable exercise but they are predominantly strengthening the rectus abdominis and the obliques. They may not be working the transverse abdominis, which is the muscle that has been linked with back pain. It is important to pull the abdomen in to recruit the transverse abdominis before commencing the sit-up. The exercises below show safe ways to do sit-ups to protect the neck.
Place the fingertips behind and on either side of the head before you do a sit up. Try to create a double chin, and raise the trunk to the ceiling rather than curling the spine too much. Two other ways to protect the neck are to use a towel to rest the head in or to place the hands behind the head on opposite shoulders

If you have no neck pain you could try the following challenging sit up on an exercise ball.