Short answer: You don't
Wrong answer: Use the $CI = get_instance() trick.
Long answer: You shouldn't actually be trying to do this. It implies that your design isn't quite right, but on the upside it's quite easy to re-engineer the relevant bits when you first discover this problem.
It's likely that the method you want to call should simply be relocated - this might mean moving it into a helper, library, model or your MY_Controller - where it can be accessed by any number of controllers.