To use a pumice stone for washing, first add the desired amount of clothing or fabric items to the washing machine or wash basin. Next, wet the pumice stone and gently rub it over the fabric, applying light pressure. The pumice stone should be moved in a circular motion, working to scrub and soften the fabric. It is important to use caution when using a pumice stone, as the abrasive surface can potentially damage or weaken the fabric if too much pressure is applied.