how to do pilates scissors exercise

Lie on your back with your knees bent, feet on the floor. Take a moment to breathe deeply. Feel the weight of your shoulders on the mat and lightly press the backs of your arms into the mat. Here you initiate the open chest and shoulder stability you will need later in the exercise.
Bring your knees toward your chest and roll your hips up off the mat so that you are in an upside down ball position, resting on your shoulders. Cup the back of your pelvis with your hands and have your elbows directly under your hips.

Extend your hips and your legs so that you are on a long diagonal. The legs are together. Your lumbar area is not flexed, it is slightly extended making this is a little bit scary position. This is the part that makes this exercise more advanced and different than just supporting yourself upside down. You will be most successful if you think of lengthening and narrowing through your whole powerhouse.
Before moving on, make sure your chest is still open and your neck is long. Drop your shoulders if you need to, and get support from the backs of the upper arms.

Scissor your legs open, equally away from each other. The tendency is to bring the overhead leg back too far, and not take the risk of extending the leg that is moving away toward the floor. Work on that gently over time.
Pulse the legs twice in the open position and switch legs. Only the legs move. The pelvis stays absolutely stable.
Repeat the scissor action 6 times.
Bring your legs together overhead and roll down with control, as you would return from roll over.
The next exercise in the classical sequence is the bicycle exercise.