Many boxers do great with herding. I have never competed in herding competitions with my dogs, but I have had 2 boxers that were good at herding cattle, my current girl Laila is one of them. Although I would never trust Laila with sheep....
I have always started with the basics of having the dog under control (leashed) and getting them exposed to the animal I want them to work with until they no longer react with excessive energy or vocalizations, then I go from there. The approaches I use really varies based on the dog's personality. I am sure there are some books that will tell you more about how to train a dog for competition herding though.
