Just a couple of videos describing the claying process. The purpose of clay is to remove bonded contaminants that are stuck to the paintwork, and these two videos demonstrate typical claying technique on a small section of paintwork. Ensure you use plenty of lubrication and no pressure when claying to avoid leaving marring in the finish.
In the videos, listen to what the clay sounds like – it makes a light grinding like sound at the start as it shaves off the contaminants, and this goes quiet once they have been removed which is the indication that the section has been completed. The clay will also feel like it is running over a rough surface at the start and when it goes quiet, it will feel smooth