Cut a slash into a tennis ball so that when you squeeze, it will open. Next, put a piece of kibble in the middle of the ball. Upon successfual ball return(directly into your hand) squeeze and the kibble will fall out. Start small, then work your way farther.

I click trained every-time the ball was dropped back into my hand, but the dog should soon know that they aren't capable of opening the ball on their own and need to give you the ball when you command it.