Here's a guide Scrubs made to show his workflow in Illustrator. Adjust the decoration to the UV-map for the part you want to add your decoration to.
Skip step 10 and instead add the decoration as shown in this video by Scrubs, To make the decoration you'll need some sort of graphics software like Inkscape or Adobe Illustrator. You can also see my workflow using Inkscape here, The Important steps for custom decoration is Step 7 to adjust the decoration to the UV-map. To make the decoration you'll need some sort of graphics software like Inkscape or Adobe Illustrator.
