Grab the weight in one hand and lay down on the mat. Keep one leg bent and stretch out the other leg (the working one, opposite to the hand with the weight). The arm with the weight is stretched in support behind your head. The other hand is relaxed on the side of your body.
From the starting position lift the hand with the weight in parallel with the stretched leg diagonally upwards until the weight touches your chin. To touch your chin slightly lift up your chest and simultaneously slightly rotate your trunk. Go back down the same way and inhale. Exhale while going up from the mat and raising the weighted hand and opposite leg. Switch the hand holding the weight and do the exercise on the opposite side.
Pay attention to breathing and neutral position of your pelvis to prevent lordotic spine curvature at the waist.
Concentrate on diaphragmatic breathing!
Execute the movement by pulling (concentrate on correct engagement of your abdominal muscles) not by jerking.
When exercising, concentrate on the correct technique and correct engagement of muscles to get the best workout.