Not hard at all, I’d think.
Dogs operate a lot based on smell, and seem to seek out ‘interesting’ ones (and roll in them, often). Training a dog like this is just enhancing a natural dog instinct.
Training them would be basically just letting them smell out things, and rewarding them enthusiastically whenever they locate a corpse (or part thereof). Dogs are trained to find drugs, explosives, lost kids, mushrooms, ducks & pheasants, deer, etc. all the time. So finding corpses would be just another smell to train for.