SkinStuff

The diffuse is pretty easy. You just create a transparent texture in Blender, map it to the body model as a transparent texture that goes over the diffuse itself, and paint the scars over it by using texture painting. Then you save that as a 32 bit png and merge it later to your body diffuse texture with photoshop or gimp.

The normal is harder though, you first have to create the scars normals, using the png you saved, I recommend using crazybump, since it has really nice features.

Then you take the body object normal map and revert the process of making an object normal map which works with Skyrim (reverse the blue and green channels, invert the blue and red channels)

Then you convert the body object normal map to a tangent normal map (map it to the body's material as an object normal map and then bake a tangent normal map out of that same model).

Then you merge the body tangent normal map you just created with the scars tangent normal map in crazybump.

Now you convert that merge back to an object normal map (the same process you did to create the body's tangent normal map, but you now map it as a tangent and bake it as an object).

Finally, you go through the process of making that normal map useable by Skyrim, by switching the green and blue channels, and then inverting the green and red channels.

I used Blender and Photoshop to do all this.