Draw the outline of your flagstone on paper (square or circular). Inside the outline, draw the design you want to have on your marker. Pick a piece of paper that is the same size as your flagstone, so you won't have to make proportional calculations for your marker later. When you are finished, draw a line down the center and across the middle to help you transfer the correct scale to the finished surface.
Place ceramic tile pieces on your drawing. The layout should be identical to what you put on the marker. Put tile glue on the back of each piece and put each piece on the flagstone where it belongs.
Wait for the glue to dry. Approximately 24 hours is typical, but check the bottle's instructions to make sure. Mix the grout and spread it liberally over the entire surface, pushing it down between individual decorative pieces. You can use a squeegee or put on some rubber gloves and do it by hand.
Wipe off any extra grout with a wet rag and allow the whole surface to dry. You may notice some settling in the grout during the next 24 hours. If cracks or hollows form in the grout, add some more and wipe off any excess. Repeat until you are happy with the grout coverage, which should be even, and either level with the tiles and decorative pieces or just below their level.
Apply sealer across all of the decorative pieces and grout once you are satisfied with the grouting. Wait for the sealer to cure, according to the instructions on the container, and then place the ceramic marker in your garden.