To generate an embossing tool:

1. create an SVG file of your design
2. edit the SCAD code to point to your SVG file
3. specify the width and height you want the design to be (you may need to play with the values to get the desired aspect ratio)
4. render the model and export as STL

Depending on the resolution of your printer and the amount of detail in your design, some of the finer details may be lost. 

The generated model consists of two pieces: a bottom plate with a raised design and a top plate with an indented design.

To use the embossing tool:

1. place the top plate, face down, on the bottom plate, lining up the designs so that they fit together
2. snap together the far end of the stems, forcing the top one into the U-shaped bracket of the bottom one (this helps keep the plates aligned)
3. pry apart the top and bottom plates, and slip a piece of paper in between
4. press down firmly on the top plate, making sure to apply pressure on all areas of the plate
5. pry apart the top and bottom plates, and remove the paper

I've been printing these in PLA with a 0.2mm layer height. Not sure how this would work with PETG, since it tends to flex. 

Printing these using an SLA printer might be interesting, since you're likely to retain more fine details in the design.