I figured out how to make scars after some trial and error and combing through various resources. The part the eluded me the most was how to format everything. Channels and DDS file exporting were not things I ever messed with despite playing about photoshop over the last 20+ years lol.

Check out this link to  HeartsWires' Random (Neck) Scars Wip file page and download the "GUIDE - merging scar replacers and scars." Here you'll learn how make your own scar packs and it works for the ones from Larian and from any mods that use the Scar atlas.
(I think the only one you don't want to replace is the illithid pattern one- the game doesn't let you manually choose that tile anyway. If there is a way to add to the number of selectable scars, I haven't found it.

Also, I've using Paint.NET to save the DDS files. I should be able to use Photoshop for this- but there are simply too many DDS options. HeartsWires shows in detail how to save the files in Paint.NET, so you're in good hands.

For my part, I use Photoshop 2024 and just realized I can easily make normal maps in it.  It's just a filter which blew my mind. The tricky part is designing the scars and figuring out the light/dark values. The burn/dodge tools come in handy for getting smooth shades of gray.

Here's a short video I made demonstrating the method  that so far has been working. It just covers the Photoshop portion and I have no idea how this might translate into GIMP or other image editors- Scar Tutorial: