Yea so i really like watching those portal gun skins. I thought it would be awesome to use it myself.
I see that somebody posted a portal 2 skin in the Other Files. How do i apply it?

Not sure if these are the best/easiest ways or not.. but they get the job done without much fuss.

You can't leave files in the same directory structures as VPK files any more, and have the game override 'em (like you could with GCFs).. but you can edit the .VPKs really easily.

First Way: (easiest)
in C:\Program Files\Steam\steamapps\common\portal 2 there's a portal2 folder and a portal2_dlc1 folder.. each has basically the same structure, i.e.
portal2_dlc1\maps, portal2_dlc1\models, portal2_dlc1\materials, etc .. so anything in the dlc folder overrides the original content... when it's in a vpk!
(i.e. it will load new files you've introduced like new maps.. but will not override anything already existing, unless you shove it in a VPK)

Simple solution, make a portal2_dlc2 folder ... then when dlc2 rolls out, go ahead and make a dlc3...

Inside that, make a folder called pak01_dir ... this is where you'll put new stuff, so you might have:
C:\Program Files\Steam\steamapps\common\portal 2\portal2_dlc2\pak01_dir\materials\mystuff...
C:\Program Files\Steam\steamapps\common\portal 2\portal2_dlc2\pak01_dir\models\mystuff...
C:\Program Files\Steam\steamapps\common\portal 2\portal2_dlc2\pak01_dir\scripts\mystuff
etc.

Once that's done, open up
C:\Program Files\Steam\steamapps\common\portal 2\bin\
and drag your pak01_dir folder onto VPK.exe.
It'll give you the file:

C:\Program Files\Steam\steamapps\common\portal 2\portal2_dlc2\pak01_dir.vpk

Done.

Not sure how this might affect the DLC2 when it rolls out... i.e. how badly it might screw up your DLC2 install.. but I'd immagine all you have to do would be delete the messed up dlc2 folder and verify your files again.
Also, you'll need the authoring tools for this.. or at least the alien swarm\left4dead2 SDK, as their version of VPK.exe will do the job nicely.

Second Way:

The pak_dir files are like an index for the regular pak_002, pak*_003,etc... but in fact, they actually contain the first couple KB of the files they index... so if it's a VMT file (the file that describes the VTF textures) it'll contain the whole file, on account of they're only a few bytes.
And they're only a few MB.. i.e. 3.6ish for the original pak01_dir.

Take the portal 2 logo for example:
/materials/vgui/portal2logo.vmt which points to /materials/vgui/portal2logo.vtf

If you open your original pak01_dir.vpk with a hex editor and search for the string:
$basetexture "vgui/portal2logo"
then you've found portal2logo.VMT
you can alter that to say
$basetexture "vgui/mynewlogo01" (make sure it's the same length or smaller)

Make a backup, save, and you're done.
The game will now look for mynewlogo01.VTF on all the search paths.

This is obviously more convoluted, and won't work for a lot of things.

Anyway, it's 7am and I should catch up on some sleep right now, so gimme a nudge if I missed anything.
Hope that helps

That was very advanced Ok ill check it out thank you