Grip bars with hands shoulder width. Keep body in good alignment (abs tight, chest up, back straight). Use your abs to pull your legs up. In a controlled motion, curl lowerbody up, moving diagonally to the left or right. Contract obliques fully without compromising form. While maintaining the controlled motion, return to starting position. Do not allow body to swing. Do not allow muscles to relax before next repetition.
Here are a few suggested alternative exercises:Ball Crunch ObliquesBall Oblique TwistBall Russian TwistDumbbell Side BendsKneeling Oblique TwistRoman Chair Side BendsRussian TwistSide Bridge