Here I mainly just used a
brush and a
color picker ^^ In a place where a slightly brighter surface and a slightly darker collide, I picked a
brighter color for the brighter surface, and a
darker color for the darker surface, and drew 2 lines next to each other, plus minor adjustments so it looks better from afar:
The brush in my case was 1px wide & 100% opacity, not sure if that's how it is in GIMP, but it's not like it needs to be exactly that obviously ^^
Keep in mind that there are most likely better ways to do this, and that you don't always need all the steps, this is just a small example on how I did that version