Place an extended magazine into the well just in front of the trigger housing.
Place one end of the rubber band on the back of the trigger and pull it forward around the extended magazine. The band should keep tension on the trigger, pulling it forward.
Raise the rifle to your shoulder and fire normally. Leave your finger in the firing postion and pull the rifle away from you with the left hand. The trigger should snap back immediately, allowing the trigger to be forced back into the firing postion by the pressure of it being pressed against your finger.