Well, I learned the basics by simply trying it on a willing lady and asking her to tell me what worked and what didn't. Beyond that, it's largely a matter of thinking about what might feel good if someone did it to _your_ feet (or hands, the principle's the same), and paying close attention to how your subject is responding. Applying firm, continuous pressure is less likely to provoke a ticklish reaction than a light or intermittent touch. Easier to demonstrate than to describe, but not much harder to learn from doing than from a demonstration. It's a useful skill, Like any ape, we're wired to appreciate grooming behavior (from individuals we trust), and like shoulder/back massage this is a form of grooming that American society isn't hung up about.