Mark your pattern on the mat with masking tape securely attached along the edges of the design. Any loose tape may allow bleeding underneath. Tape any stencil to the mat with the stencil interior edges as flat as you can make them.
Paint your pattern on the sisal with outdoor latex paint. Sisal quickly absorbs the paint, so layer it on somewhat thickly and work it into the spaces between the fibers.
Let the paint dry completely before adding a second coat of paint. Add an area that touches another painted area only after the first area has completely dried. Otherwise the paints will run together. Tape the painted area well to ensure the second paint does not leak under.
Allow the rug to dry for 12 hours before using it.