It is used after washing the car.
The procedure is simple, you rub in onto the surface of the car, whilst also spraying or using any anti-friction detailers liquid, either supplied with the clay bar kit, or seperate.
You either rub the entire car, which is a hefty procedure, or the parts which you think need more care than the rest, such as the front and the back end.
You rub it on in different patterns until you feel that the clay bar is sliding over the surface and there is not surface traction.
But you have to be careful to keep on folding the clay bar if it catches too much contaminants, sometimes you may even have to seperate that dirty part from the clay bar, and if you let it slip from your hand and fall on the ground, you're in big trouble, it would attract a whole lot of dust particles and tiny stones and what not, which might scratch your paint finish.
so you have to be extra careful whilst using it.