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Simple way to make a skybox thanks to Philip[phb5000].

Making a skybox is really simple:

1. Make a cube
2. Hollow it
3. Apply the "Skybox" texture
   

Now you have to apply a sky that will be used in the game. For some reason, even if you arent including a sky in your level (like an indoor level) you still have to set the sky texture. The default sky texture is one that, for some reason, doesnt work when I try to compile a level.

To set a different sky texture do like this:

1. click "Map" menu
2. click "Map Properties"
3. select the "SkyBox Texture Name" key
4. change the value to a different sky texture like "sky_dust"
   

Tip: if you cant find tutorials for CS:S, just try looking for hammer tutorials for like CS. There is a lot of stuff that is the same.