The mod itself replaces the "Black T-Shirt" that can be bought at Pro-Laps.
You need a certain tool like TXD Workshop in order to import the .txd file into "player.img" which can be found at "models" folder. It's pretty simple!

Tools used:
TXD Workshop (Exporting the vanilla tshirtproblk.txd file and then importing the edited version of it)
GIMP 2 (Editing)