Sit upright on a stable chair or wheelchair with your back supported and a table close in front at elbow height.
Ensure both feet are placed flat.
Your assistant can help you place your forearm and hand onto a table in a comfortable position.
Your assistant should use forefinger and thumb to support either side of the selected finger joint, whilst supporting the hand.
Your assistant can then gently bend and straighten that joint, gradually increasing the comfortable movement possible.