1) Obtain a clear, plastic container. It's important that it's clear because you will want to be able to draw the image in the front. In this example, it may not be as important but if you want to etch letters or numbers, it will be a little difficult to write it out backwards in the interior of the mold. The container must be slightly flexible and easy to cut and peel off the pot. Step 6 will explain how I came to this conclusion.
2) On the exterior of the container, use a permanent marker to draw the image you want imprinted.
3) Use a glue gun to outline the drawing from the interior of the mold. You may want to use low-temp glue. I used hot glue and the heat warped the container a little bit.
4) Working from the bottom up of the container, apply the concrete mixture, pressing firmly against the walls. I really recommend using concrete rather than hypertufa for this project. The aggregates in concrete are much finer, which is needed to get into the negative space of the imprints. The perlite or peat in hypertufa might leave unwanted divots.
5) Let the concrete set for about 24 hours.
6) Cut the container and peel it off the mold. Despite greasing the interior, I couldn't pop out the pot, because the concrete wedged in the imprints locked it in place. I ended up cutting and peeling off the container. This is why, in step 1, I recommend that the container be slightly flexible and easy to cut.
7) Drill a drainage hole. You might need to use a carbide bit depending on how solid the concrete is at this stage. In this project, I had to use a carbide bit because the standard bits were not sharp enough to get through the concrete.
8) Soak it in water for a couple of days to leach out any lime; changing the water out each day.
9) Let it dry and then paint it. I used acrylic patio paint. I highlighted the imprints with colored paint, and then went back and whitewashed the whole pot.
I hope you get a chance to try this project! Please post photos on my Facebook page and share your experiences: https://www.facebook.com/BalatongTNC