[pervipete]

I suppose it's like any other thing you train a dog.

From what I remember from stuff I've seen on TV, it's basically the same techniques whether you are training one to fetch, find drugs or bringing down a suspect, it's about reinforcing the behaviour, I dunno whether you would have to make it more aggressive, or whether, as IvoryBeast pointed out, the dogs own natural mating instincts would kick in.