Prime the pot with a primer appropriate for the material. Follow the instructions on the primer container. Then, create a harlequin pattern on your pot. Draw the pattern in pencil, or you can purchase ready-made stencils in the paint department of your local The Home Depot store.
Tape off the lines with painter's tape. Trim off the excess tape with your hobby knife.
With a paintbrush, paint over the tape to create your diamond pattern. Don't use too much paint, though, or it will bleed through the tape.
Once the paint is dry, switch the tape to the outside of the pattern, and fill it in an artist's brush.
Then simply remove the tape, touch up any imperfections and enjoy! If you plan to set the pot outside, seal it with a water-based, exterior grade polyurethane. Use two or three coats.