For a different look and texture, use pebbles in your birdbath mosaic. The instructions are the same; just be sure to work carefully while grouting to fill the spaces between the pebbles.
Arrange some mosaic pieces in a section of the flowerpot saucer to come up with a pattern.
When you're satisfied with the design, use a piece of cardboard to spread tile adhesive over a small area of the saucer and arrange the tiles atop the glue. (The adhesive dries in just a few minutes, so you'll need to work in small sections at a time.)
Continue this process until the saucer is covered by the mosaic. Allow the adhesive to cure for at least six hours or overnight.
With another small piece of cardboard, spread grout over the mosaic, making sure to work it in between all of the tiles.
Let the grout set according to the package directions, then wipe the surface of the tiles clean with a damp sponge. Use a rag to remove any residue.
Apply a coat of grout sealer to your design and allow it to dry.
Set the saucer on the overturned pot. If you like, glue the two pieces together using the tile adhesive.