Sand the rough spots on your clay pots with a piece of 120-grit sandpaper in circular motions so you won't have any protruding pieces that interfere with the design. Wipe the surface of the pots down with a wet, lint-free cloth to remove dust or any dirt.
Brush on a coat of oil-based polyurethane on the insides of each clay pot to protect the paint and the pots themselves from water, which will soak through the clay to the outside. You don't need to apply any polyurethane to the outsides. Wait about 12 hours for the polyurethane to cure completely.
Apply acrylic paint all over the outside of each pot with a sponge brush. You can paint the same color on each pot if you want them all to have the same pattern, or you can apply a different color to each pot, like a rainbow effect or alternating colors. Wait about 2 hours for the base coat to dry before adding any designs with a thinner paintbrush.
Brush or spray on a clear acrylic varnish over the paint to protect it and give it a shine. You can also use a varnish with a matte or satin finish. Wait at least 12 hours before stacking your tipsy pots and adding the soil and plants.