Put your custom skin (rez file) into the "No One lives Forever 2 folder".
Do not put this into the custom folder. If you put it in the custom folder
it will not work for MP.
After you have the file in the No One Lives Forever 2 folder go ahead and
start the game. You will see this interface, select "Options".
Do not select custom or else this will not work in MP.
From here you will need to type in the name of the rez you wish to use in
the "Command-Line" box, for example; -rez deathpack_01.rez. Note
you can add multiple rez files here but be careful what you add for MP playing
or the game will not load.
Next check the "Always pass command line arguments to the game" box.
Then Select OK. Now you are ready to play. You will see your new textures
but no one else will.