It's really easy actually!
It should be better when the new ea graphics editor will work I guess...
I need to redo it because it didn't come out quite right, but I made eric williams tattoos!
I went to xplrcmn, extracted that viv, then took skn15.fsh since it was the appropriate color for williams.
from there I just dumped out the .bmp file in there, pasted the tattoos right on the arms which are the 1st and 3rd columns. Then using fshtool re-imported the .bmp file into skn15.fsh. Then I renamed skn15.fsh to erwilli_acc.fsh , slapped it into /render/xplayer/erwilli ... and presto, it's in the game!!
http://www.geocities.com/bigfrogg2000/ew.jpg