Stone, in general, is not quite milled the way metals, plastics, or woods are. This is because cutting stone doesn't produce shavings, it produces dust and potentially random chunks of stone. With this in mind, if I were to try milling marble, I'd probably look at diamond tipped grinder bits and would start at extremely slow feeds with the goal of rubbing the stone away instead of cutting it. I'd also want to run water on the cutting surface, as the amount of friction is likely to generate a lot of heat. That being said, depending on the complexity of your case, you will likely run into problems with thin parts of your design, so shallow cuts are going to be your friend.
Anyway, with a bit of research this is almost certainly something you could do on your own.
