1. Download KOTOR Tool.
2. Open KOTOR Tool and find the face you want (in ERFs-->P).
3. Extract the file as a .tga.
4. Open it in any image editing program.
5. Edit it to your desire.
6. Save it as a .tga and put it in your Override folder.
To expand on this (at least for TSL), in KotOR tool go to the ERFS and expand that, then expand texture packs, then expand swpc_tex_tpa.erf (this is the high quality textures and in my opinion the only ones worth changing). Then as JCarter has said go to the P section, an example would be PFHA01.tpc (P=player, F=female,H=head as far as I can tell) extract that as a .tga, edit, then when happy with your changes save the file in your override folder (as the extracted name) and see how changes have effected the skin. PMHA01 would be the first male head.
Remember the search function is your friend, most questions you will have will already be asked, and this section will be useful for you;
http://www.lucasforums.com/forumdisplay.php?f=595)
Happy Modding :)