Scars take a lot of practice (I'm not too good at them) but I have a way of doing it?
- Make a new layer and draw a thicker version of the scar's shape
- Erase it so it looks like fur is covering the scar (since the fur on an animal flows in a certain direction, any missing patches wouldn't be completely bare). The erased parts should follow the direction of the fur texture you're using, so it looks like it fits.
Then I usually just mix in and play around with various shades of red in an attempt to make it look like it fits.
The newer the scar, the more saturated the red will be -- sometimes in place of a scar, fur may even grow back in completely white.
BUT yeahh it's not really solid haha I'd show examples if I still had any, but I got rid of them a while ago.