In the steam credit card form, you type only the real details, or else it wont work. But as you are typing, sometimes click the taskbar (the gray windows bar at the bottom of your screen) and type random stuff, and then go back to the steam form and type in some more real stuff. That way you fill the form in, but if everything you type is copied to a log, nobody who reads the log can work out your real information, because in the log your real information is hidden (in small pieces, if you frequently switch between the taskbar and steam), within a long string of randomness. Understand now?