Set up: Seated on a fitball or chair, feet in front of your knees approximately hip-width apart. Spine should be in neutral with shoulders back and down.
Keep upright but relaxed. Lift pelvic floor (link back), gently draw in lower abdominals and hold whilst you continue to breathe normally. Make sure you are breathing into your sides (avoid chest and belly breathing) and keep your body relaxed, avoid tensing through legs, glutes and torso. Raise one arm up to the ceiling whilst maintaining core activation and balance on the ball. Slowly lower your arm with control, repeat with other arm.
Progress this exercises by bringing your feet close together, this will challenge your balance and core.