Prepare the ball. Lie on your back and put your legs on the ball with your heels and lower legs leaning on the ball at hip width.
From the starting position lift your pelvis upward. Simultaneously with lifting your pelvis, bend your legs at the knees and curl the ball below you, pulling your legs into a bridge position with feet flat on the ball. Your knee angle in that position is about 90 degrees. Your shoulders are pressed against the mat. Go down from this position in the same way but do not lay your glutes completely on the mat. Return to the starting position (lying on the mat) only after you have done the required number of repetitions of the curl. Exhale with the movement up and inhale going down.
Pay attention to correct position of the pelvis when moving upward to avoid over-bending at the lower back.
Pay attention to correct position of your feet and a sufficient angle at the knee joint. In the upper position the feet press against the ball over their full surface.
In the beginning you can exercise with your trunk supported on an elevated platform. Choose the platform tilt in line with your fitness level.