How to make acrylic paint sticks to baked polymer clay?

Im using Kato clay. Im having trouble making a solid paint color on my cured clay. The label on my acrylic paint says "acrylic gouache". I have never used them before so I don't know what Im doing wrong. When I apply the paint on my clay, the color moved around and is not sticking to the clay. Do I need to put some glaze or medium on the clay first? Please help this helpless amateur!!!