Take them ratting a bit first to get them a bit more hard mouthed and used to things having a go back then gradually start by finding the smallest weakest looking foxs then build your dogs confidence and knowledge of what its dealing with and then move on from there. No good slipping a soft mouth lurcher used to retrieving rabbits live to hand into a big strong dog fox, all your dog will do is treat it the same as a rabbit pick it up barely squeezing and then WOLLOP the fox has turned in your dogs jaws and took his nose off or an eye out or one of his ears off ive heard of it happening, but yh