I realize your asking in generalizations, so no photos available. It depends on the shape and size of the part. Some parts can be clamped to your lathe faceplate or mill table, possibly with shims in strategic places to get the first surface as 'flat' as possible. Then you mill or turn that surface flat. Then, all other surfaces can be referenced to the first one. Sometimes you need to use an angle plate to hold the part, but you need to be careful how you clamp the part to that - no loose clamp arms sticking out to snag anywhere.
If you are using a four-jaw, again you may need to use a shim to even out a taper in the casting. Just make sure the shim can't suddenly come loose and let the whole thing loosen up. If the casting allows enough extra material, file or grind the clamping areas parallel for best holding in the jaws.
I know this isn't the clear tutorial you would like to see, but I've never done one. I'm just speculating on how I'd do it if I were setting one up. Now the guys who really know can pick up the thread and clarify my start.
