What type of ball do you start with-what size is it???
We started with a soccer ball actually. Once they had the idea with the soccer ball we used the full size exercise balls (45cm) until we could find the smaller balls.
If you have a Five Below store near you they've been carrying kids sized exercise balls for $5 (these are the right size for Shelties though learning to push the big ones is a good idea also). I believe they have soccer balls for $3.
I'd also suggest getting a yoga mat (also available at Five Below for $5) and cutting it in quarters to make a targeting mat. We started sending the dogs to a large mat (no ball present) in front of us and slowly backed away from the mat (keeping the dog beside us) and sending them to it. As they got better, the mat got smaller until it was about the size of a coaster.
After the dogs started getting the idea of going to the mat, we added the ball between us and the mat so they'd go behind the ball. We worked this skill at the same time as the go to mat without the ball but, as I said, they had the idea to go to the large mat down about 80%. Basically you are repeating the mat training without the ball WITH a ball in place.
Another skill is teaching them to always stay directly opposite you and push the ball with their nose. (Use of feet and teeth are point deductions.) We held the ball in front of us and told the dogs to push with a treat under the ball so they'd learn to put their head down to push.
That's probably enough to get you started! I'll see if I can find the electronic copies of the training materials I have.
Did you get a book, and if so, which one did you like?
Unfortunately there isn't a book out there. We figured out how to train it on our own (the Treibball committee at my agility club) based on the knowledge of what we needed our dogs to do. The ATA (American Treibball Association) doesn't make their training materials available until you are a paid member; which IMO is a dumb rule. If they made the basic training material available maybe more people would be interested in it. Let the Intermediate and Advanced information be member based if you want. You need to GROW the sport and restricting learning doesn't help it grow.
There is another Treibball organization in the US,
NATE. I don't know much about them but maybe there is other information there.