Dab the design surface on a paper towel to remove moisture. To create a firm base on which to stamp, slide a piece of stiff, flat material underneath the fabric or item to be stamped. Cardboard and foam board work well for this purpose. Brush the paint onto the potato and stamp the design on the desired areas. Experiment with other vegetables.
Carrots make good dots, and the cross-section of a celery stalk works well for creating roses.
Be sure to use a washable paint when stamping on fabric. Brush the paint onto the potato instead of dipping the potato into the paint.