You don't necessarily need to do anything but replace anything but the axle and add a skewer. For a given brand the threading and diameter are the same with some rare exceptions.
http://sheldonbrown.com/harris/axles.html
The q/r axle needs to be a couple mm's LESS than the current distance between the locknuts on the hub plus the sum of the thickness of both dropouts (including the derailleur mounting plate if you have one. Once you have the axle length you just need to obtain a skewer compatible with that length.