To avoid having the ink bleed out when coloring, be careful not to stop or pause with the tip of the fabric 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 grain of your fabric also has an impact on how well a fabric marker works. A coarse grain or rough texture to a fabric means there are possible “bumps” the pen has to go over. A fine grain or smooth fabric is easier to work on. If in doubt, test the marker on a scrap bit of fabric or in an inconspicuous area.
To prevent ink from bleeding through the layers of the fabric, insert a shirt board or piece of cardboard between the layers of fabric to prevent the color to seep through fabric to the second layer of fabric as well as provide a stable decorating surface.