Sit in your wheelchair with your feet resting on the footrests with a solid object such as a table in front of you.
Ensure the brakes are applied.
Place your arm on the table in front of you.
You may wish to use a cloth to reduce friction.
When ready, sweep your arm across the table in front of you keeping your elbow straight.
Next, sweep your arm across the table away from you keeping your elbow straight.
Return to the start position, and repeat.