It sould be pretty straight forward, you got a positive and negative when soldering it on. All battery holders should be the same on any motherboard that has a coin battery older (PP3 I think they call them) so I hope that is clear.
You dont have to buy a whole non-working motherboard just to get one either, they are avalible seperately from a good electronics store (such as Tandy, RadioShack, Maplins for example depending were you are in the world).
