Different depths are available, we find the 0.047 relief depth (K152) is best. While shallower ones will stamp a crisper design, it will be difficult to avoid sponging pigment out of the recesses when cleaning the high spots. Traditionally polymer plates have had metal backing (and were brittle), but these are flexible and inexpensive. When designing them create a border around the outside. This is important because when the stamp is pressed hard into the clay, the edges smear outward, that containment-line keeps outside design edges clean. Also, the plates do not actually need to be stuck to a piece of wood, it is better to lay them face down on the clay and use a wooden block to press them into the clay (which needs to be quite stiff). Because they are flexible it is easy to peel them out. Use spray cooking oil as a parting agent if needed. The cost: A 2"x3" stamp cost me about $50 (with shipping) in Aug 2020. But, as I was checking out, their site informed me that the minimum charge is $35 and that I could have ordered 40 square inches more for the same price. They accept PDF and bit image files and the shopping cart enables previewing. The cart might generate CMYK plates (four of them for process color printing), just remove the CMY ones and keep the K (black). You likely will not get it right the first time. The most common mistake is having too much detail or too small printing. It is best to make your images using vector graphic software like Illustrator or Inkscape. Another common mistake is to forget to make them reverse-reading (horizontally flip them).
The buff stoneware mug is fired at cone 10R and celadon glazed. The recesses were colored with a tenmoku glaze (on bisque by painting it into the recesses and sponging away the high spots). An outer containment line on the plate prevented the outside line from smearing outward and it provided a definite profile for cut-out after stamping.
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.
Knowing how thick to make the lines and how much detail will be shown is tricky. This is a letterpress plate from boxcarpress.com. In this press the characters are about 5 mm high. The stroke width is 1pt. These are Aramaic characters from the Moabite stone. The complete enclosing rectangle is about 2 1/4" by 3".
Boxcar Press website
They make letterpress plates (or polymer plate) stamps. Online job submission and ordering. You pay by square inch.