Painting on fabric is not like painting on paper or canvas. Fabric tends to move with the brush unless properly secured and the paint tends to saturate at a different rate. That being said, you need to practice and practice some more before you try painting on your final item. How you do that is
to get some fabric to practice with. Old t-shirts, pillow cases, cut up sections of bed sheets all make good practice fabrics. There is no need to run out and buy practice fabric, just use what you already have.