LOL...ok,basically you go and download a hand skin you like,open hlmv,go to the textures tab,find the hand texture in the list,click import and find the bmp you just downloaded(the hand skin)and open it.Make sure they are the same size though(for example,both 512x510 pixels)otherwise you will get an error message,of course,theres a way around that but i couldnt be bothered explaining,considering you dont know how to use hlmv you wont get that...
Haha i had to figure this out by myself T.T
Well put your hand skin on your desktop.
Open up HLMV and open a .mdl file of a gun. Download in this site.
Go to the textures tab.
Open the scrollbox and find the one that looks like a hand.
Click import texture and select the hand skin you have.
Save the .mld file as the same thing you had before.