How does one break long bones for broth?
A normal hammer (one that is used for nails) will not suffice for breaking relatively strong bones, such as the leg bones. 
Personal experience: I bought a pork knuckle, and attempted to break the bone with a hammer (600g head), and it resisted 5 minutes of straight hammering, despite the fact that the surface was significantly dented, and many bone fragments were scattered about the room. 

A few online searches suggested that a sledgehammer would be required to break bones for broth. 

In short, get the butcher to do it for you. You probably don't have the necessary equipment needed to smash bones.