I don't know if you still need this, but... With Valentine, I started by getting her to take the ball. Then trading her for it with treats. This got her excited about the ball and also taught her drop it. Then I would toss it a little ways (not too far) and call her to me in a very excited voice. I would give her a treat when she got to me. This also got her to drop it as soon as she got to me.
She is now fetch obsessed and very skilled at Drop It.