Sand the outside and inside of all terra-cotta pieces to remove rough edges. Clean the surface of the terra-cotta under running water and allow them to dry completely.
Paint the surfaces of the pots using acrylic paints. Paint them solid colors or your own design, whichever you prefer. Allow the paint to dry completely.
Place the 16-inch terra cotta pot upside down on your work surface. Place the 14-inch terra cotta pot upside down on top of the 16-inch pot. Apply plumber's caulk around the inside lip of the 14-inch pot to secure it to the larger pot. Place the 12-inch pot on top of the 14-inch pot in the same manner. Apply plumber's caulk around the inside lip where the two pots meet.
Place the 20-inch terra cotta saucer, right side up, on top of the terra cotta flower pot stack. Apply plumber's caulk around the bottom of the 12-inch pot and press it against the saucer to secure it. Allow the plumber's caulk dry completely.
Coat the outside and inside of the bird bath using a water-based polyurethane sealer. Allow the sealer to dry completely.
Place the bird bath in your garden. Add water to the saucer.