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From HalfLife 2 Knowledge Base

List of steps on how to convert your 2d skybox (basically your sky as a 3d skybox doesn't include that) over to source.

1. Prepare your tga files (i.e. skynameBK.tga, skynameFT.tga, skynameUP.tga, skynameDN.tga, skynameRT.tga, skynameLF.tga) or if you dont have tga's convert your images over to 32bit tga.
2. Convert tga to vtf using VTEX.exe (this will create a skyname--.txt which you will later edit then rename to skyname--.vmt)
3. Once done converting all files from tga to vtf go into the text documents and put this in:

"UnlitGeneric"
{
// Original shader: BaseTexture
"$basetexture" "skybox/skyname"
"$nofog" 1
$ignorez 1
}

• Replace skybox/skyname with skybox/(your sky)
• Dont forget for each txt to add the appropriate image tag (i.e. bk, ft, rt, lf, up, dn)
• Then rename the file to (appropriate name and tag).vmt
• Now go into your /valve/steam/steamapps/<yourname>/counter-strike source/cstrike/materials folder and create the folder skybox.
• Now copy and paste/cut and paste/drag and drop your new vtf and vmt files into the skybox folder.
• Then before you compile your map go to Map>Map Properties>Skybox Texture Name and type in your skyname without the tags (bk, ft, etc)

I hope this helps.

Tats...

Thank you Qasw as I don't think I would have ever gotten that last piece of the puzzle if not for you