1. Open TSR Workshop
2. Under preferences, you'll find 3 boxes. One for TS3, one for WA, and one for HELS. This is where you tell TSRW where your game files are.
3. Pick the HELS folder. Tell it the folder is ElectronicArts/The Sims 3 Ambitions. This makes TSRW load the ambitions data, even though there is no option to direct it to your ambitions folder.
4. Go to Clothing and find a tattoo to clone. Be sure to put something under "unique identifier" when prompted to change the text info.
5. Go to Textures -> Core. This is the tattoo. Export it like you would anything else, and edit it like you would anything else. This one file contains the shape and the color masks, just like a pattern, so if you can't figure it out, look at a pattern tutorial. The Alpha is the tattoo shape, and the color masks are done just like a pattern. You will likely have to do all from scratch. Be sure to allow color options under "MaskInfo".
6. Save and check it out in-game. To export as a package, to to Edit -> Project Contents -> Export -> To Package.
Now, mine doesn't change color, and I'm not sure why yet. I'm still working on figuring this out, but at least the basics are available now...