To start: Grab a medicine ball that’s between five and 15 pounds. Stand with your feet shoulder-width apart, making sure your knees are not locked. Then try one of these two variations.
For stability: Start with the ball at chest level, and slowly write the cursive alphabet in front of you. While your arms move around in all sorts of angles, you need to use your abs to keep your torso still. Think of keeping the area between your ribs and pelvis solid.
For mobility: Start with the ball at chest level, but increase the range of motion of your arms making your alphabet larger, using your torso and rib cage to write the letters as well. Keep your abs engaged the entire time. For more of a challenge, try this version in a squat or a lunge.