Stand in good body alignment (abs tight, chest up, back straight) with feet in comfortable position. Hold dumbbell in one hand. In a controlled motion, start lowering body to same side dumbbell is held, to comfortable position. Once you stop, bend the opposite direction to contract abdominals/obliques. While maintaining the controlled motion, continue left and right to desired number of repetitions. Do not allow muscles to relax before next repetition.
Here are a few suggested alternative exercises:Ab Crunch ObliquesBall Crunch TraditionalLateral Bend with Barbell