Halter training, as in putting a halter around its head? I would seriously recommend that you not attempt this. This could easily result in the duck panicking and struggling and injuring itself or maybe even snapping its own neck while trying to get out of it.
Oh good, a harness is a totally different thing! A halter goes over the head of an animal. I don't have experience with it, but you might ask in the house duck thread - someone in there probably knows
Sorry if I sounded harsh earlier, but I have been reading a lot of crazy things on here lately. Someone wanted to put a muzzle on their duck to walk it in the park, for example. They actually wanted to know if they could use string to tie its beak shut.