BackintheSaddle, when I used my breast collar harness to start my horses, I also had an extra singletree that I attached to the tire or wood pallet that I dragged around. You can see the singletree in the picture that smbrobs showed with the traces (the tugging straps) connected to the heel chains, which are attached to the ends of the singletree. That keeps the chains spread out behind the horse, so that the horse doesn't step on them. The first time or two that I hitched up to a training load, I actually used baling twine to connect the traces and the singletree, tied it in a quick release knot and kept the long tails close to hand. I wanted to be able to give a quick yank and free the load we were dragging if we had a wreck and got tangled in the harness. You have to get really close to their back legs to connect up or release, and that was NOT a place I wanted to be if my horse started running or kicking out at the load!
It's a lot of fun to drive, and if you've done ground work already, the training is easy. You'll enjoy it!