This crest was made by rolling a thin slab of clay, allowing it to stiffen, oiling it and then pressing it using a letterpress plate (and a wooden block and hammer). The plate had the oval shape containment line, so cutting this shape out on a turntable was easy. A slip was applied to the back of the slice, and because it is so thin, it softened almost immediately (because of the wetness of the slip). It was then applied and pressed into place on the leather hard mug. After bisquing, an iron oxide slip was applied and then sponged off. The clay fires buff color, but it's texture retained enough iron to stain it leather brown. For certain clays, the fired surface looks remarkably like real leather.
Different depths are available, you need the 0.047 maximum relief depth (you can order a sample pack to try the various types they have). While shallower ones will make a crisp design into the clay, if you wish to put color into the recesses (at bisque stage) the shallow depth will make it difficult to avoid sponging it out when cleaning the high spots. Traditionally polymer plates have had metal backing and were expensive (and brittle). But these are flexible, inexpensive and easy to get online. When designing them create a border around the outside (when the stamp is pressed hard into the clay, the edges smear outward, that containment-line keeps edges clean). Also, the plates do not actually need to be stuck to a piece of wood, it is often better to lay them face down on the clay and use a wooden block and hammer to press them into the clay (which need to be quite stiff). Use spray cooking oil as a parting agent if needed.