Create a Watermark
As far as I’m aware, Darktable will only take *.svg files for watermarks. I used the free Inkscape software to do this. I’m not very fancy, so I just created some text that says my name in a fancy font. In order for this to appear properly in Darktable, you must convert the text to a path. To do this, select the text and press SHIFT+CTRL+C. You will no longer be able to edit the text as text; it is now a collection of nodes and connecting arcs. I tried saving the SVG file without converting the text to a path and ended up getting a black box instead of my signature. I also made sure to resize the document to fit the text.
Inkscape: Resize Page to Content
To do this, Go File->Document Properties and expand the “Resize Page to Content” section. I put in some small margins around the text, but this isn’t necessary. Click the “Resize page to drawing or selection” to shrink the page size to fit your watermark.
Finally, save the watermark as an SVG file.
Move Watermark To Directory
Darktable looks for watermarks in a specific directory. On my Mac, it looks in ~/.config/darktable/watermarks. The watermarks directory didn’t exist, so I created it and then moved my watermark file into the directory. For more information about Darktable’s watermark settings, check out this part of the user manual
cp ~/tim6.svg /Applications/darktable.app/Contents/Resources/share/darktable/watermarks/