I imagine it would be a flat scar. Its for Saix from Kingdom Hearts http://i226.photobucket.com/albums/dd287/bigkitty91j/Kingdom%20Hearts/Saix-2.jpg
oh that's easy, you can do it many ways. But first i'd apply a thin coat of liquid latex in the X shape let it dry (hairdryer helps dry it faster) then go from the middle of the X and add another layer making sure to stay really thin near the edges so you can blend in the latex to the skin. (Makes it look more real) Let it dry and just do this over and over till you get the thickness or look you want. If you want an indent to make it look like it has taken out some flesh, just lightly press the latex with something (paint brush end, fingernail, etc.) and let it dry it should do a indent if not. Try scar wax, scar wax is something you would want to put on before the liquid latex or else it becomes a mess. Then just take some different shades of red (dark for the inside and the lighter for the outside) and just paint over the latex.(makeup works, acrylic paints work, 3d puffy paint works real well)
Or you can draw it on with makeup and skip to the whole liquid latex part.
Sorry if this is confusing i'm better at showing then telling XP lol