Their are many dog training books that discuss the aspect of force fetch retrieving. This is the exact purpose for which it is intended, even though some people think all dogs should be force fetch trained. Keep in mind, when it comes to force fetch training, you cannot stop until the training process is totally complete, otherwise risk ruining the dog. I reccommend you hire a professional trainer to do this for you, in order to get the best results.
Breeding and Training Boykin Spaniels, a True American Gundog