Sand a large, terra cotta saucer with sandpaper. This removes any uneven surfaces that make it difficult for you to attach your mosaic pieces. Wash the saucer under running water to remove any dust. Allow the saucer to dry completely.
Seal the surface of the saucer using a polyurethane sealant. Allow the sealant to dry. If desired, paint the saucer with patio paint prior to sealing. However, if you want a natural look to the outside of the birdbath, skip the paint.
Rinse the mosaic medium under running water. Dry completely. You can use any medium you would like, such as glass beads, mosaic tiles or small pieces of mirror. Another medium idea is old dishes that are chipped or broken. If your mosaic medium is large, such as ceramic tile, break it up before washing. Do this by placing the tiles in a pillowcase and tapping the tiles through the pillowcase with a hammer.
Trace the bottom of the saucer on a piece of paper. This gives you a place to design your mosaic. Place the mosaic medium on the traced circle, arranging the pieces until you like the design. Leave a minimum of 1/8 inch between pieces. This helps ensure the pieces adhere to the inside of the saucer properly.
Apply a PVA adhesive to the back of the mosaic pieces with a paintbrush. Place the mosaic pieces into the saucer in the pattern you designed. Allow the PVA adhesive to dry a minimum of 24 hours.
Prepare a grout mix according to package instructions.
Moisten the surface of the mosaic and the saucer with water. You want it damp, not soaked. Apply the grout to the wet mosaic pieces. Spread the grout around, pressing it between the mosaic pieces. Wear gloves and press the grout in between the pieces, if necessary.
Allow the grout to dry for 20 minutes. Moisten a sponge and wipe the surface of the mosaic pieces to remove any excess grout. Wipe the surface of the mosaic with a dry cloth in a circular motion. This removes any film and shines the mosaic pieces. Leave the grout to dry completely. This usually takes 2 full days, but follow the directions on the grout.
Place the finished mosaic birdbath on the ground in your yard. Alternatively, place it on a store-bought garden pedestal. Another idea is to stack three flowerpots inside one another to form a pedestal. Use a 16-inch pot on the bottom, a 14-inch pot in the middle and a 12-inch pot on top. Apply plumber's caulk to the inside edge of the pots to secure them. Turn them over and place them in your garden. Paint them and seal them using patio paint and polyurethane sealant to create a stand. Place the completed mosaic birdbath on top of the stacked flowerpots and secure it using plumber's caulk.