The idea behind breaking scents is to make the scent something that your dog smells all the time. That way when he runs across it in the field he has no interest in following the smell. The goal is to make your dog bored with the smell of the animal.
Keep a piece of scent-soaked cloth, sponge, or a scent pad attached to your dog's collar. When I used breaking scents on my bird dogs, I just tied a piece of an old wash cloth to the dog's collar. We refreshed it every day for a couple of weeks. Some dogs require more time.
The downside to breaking scents is that they will not stop a dog from sight chasing, but they will stop the interest in following a rabbit's trail. If you are having issues with your dogs running Fox when they see them, you really need to go with a remote training collar. I recommend these units:Garmin Tri-tronics PRO 70
Garmin Tri-Tronics PRO Trashbreaker