how to do the open leg rocker pilates exercise

Begin with a spine stretch. This will help you center yourself, and stretch out your spine and your hamstrings. Sit up tall on your sit bones and extend your legs out about shoulder width apart. Flex your feet. Avoid locking your knees. Inhale and stretch out through the sides of your back through your fingers. Exhale and reach forward about shoulder height (or touch your toes), curling your spine. You may also touch the floor between your feet. Then release and continue.
Bend your knees and pull in your abs. Reach to grasp your ankles. If that is difficult, you can also hold onto your calves.

Lift and extend one leg, balancing between your sit bones and tailbone, keep your abdominals activated.
Lift and extend the other leg. Your legs should be shoulder-distance apart—there is a tendency in the exercise to allow the legs to be too wide, but keep them about shoulder width apart. Draw in the abdominals as much as you can and maintain balance.

Inhale and roll back. On an inhale, use a deepening scoop of the abdominals and the fullness of your inhale to propel your roll back onto your shoulders. Don’t roll too far back onto your neck or head. Stay in your C-curve as you roll, leaving your head and neck off the mat.
Exhale and return. Remain in your C-curve and use your abdominal muscles, along with a strong exhale, to bring yourself back to an upright position. Hold here and balance.
Repeat, rolling back and returning while inhaling and exhaling as described.