Be careful not to stop or pause with the tip of the marker resting on the fabric as the color will bleed out into it. If you find yourself hesitating, lift the marker off the fabric while you think about what you’re doing. The tricky bit is that you need to work quickly so the color doesn’t dry on the stamp, but that’s easy to do if it’s a small stamp. You can, of course, use multiple colors on a stamp, not just one. Pressing the stamp down a second time will give you a lighter image as there’ll be a little color on it. Experiment on a scrap piece of fabric to get a feel for it before doing it “for real.”