I have downloaded a pack with a lot of custom stuff, and I've seen that in the instructions it says that to install the skins I need to overwrite some files in the iw_06, iw_07.iwd and iw_11.iwd from the main folder. The thing is that I have tried to do it as I have seen at here, creating a z_something.iwd and putting all the stuff in it, but it doesn't work.

game_folder\main\z_stuff.iwd\images\something.iwi

If you want my advice, don't mess with the stock IWDs

That's what i did, but when i enter in a game, it shows the original skin, not the one that I put in the z_something.iwd

I have replaced the original files in the iwd and it seems to be wornking on both private and server so I will leave it like that for now.