It also depends on weather or not you're working with meshes or with primative shapes. In either case, it should be fairly simple- nine times out of ten anyway. Just get a drawing or picture of the prop you want, figure out the basic shapes that go into it's appearence and then combine the primatives - if you have the right ones- into the shape. Sometimes you might have to export the assembly and re-import it if you're doing alot of adding-on. not to blow my own horn, but you might check out the modeling gallery or my gallery and take a look at the train I'm working on. I'm building it entirely in Poser out of Poser primatives and primative sets I've tracked down. Might give you some idea of a direction to take.