Wrist supination/pronation (weighted object)

Sit upright in front of a table.
Place your weaker arm onto the table with your elbow bent.
Hold a rolling pin or similar long object in your hand, holding it at one end.
Turn your wrist and the object over so that your palm faces upwards.
Then turn your forearm over with your palm facing downwards.
Aim to keep your elbow, shoulder and body still during the exercise.