This is not the thread you are looking for
Seriously, the .vmf isn't any better. People have to download it and if they don't see the problem right away even compile it. One picture might get you the answer instantly from a person just surfing by.
I will tell you some of them later, but let me tell you a few tips:
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First of all, never texture the walls of anything with the skybox textures themselves, only use the toolsskybox texture.
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I see you want to make a whole environment and then use it as a skybox with its props and stuff, right? The basic idea behind this method is: to build the whole map first into a perfect square-shaped room (take into account that you cannot simply use whatever stuff you want... only certain props, certain textures and mainly brushes are allowed), then scale it (this is why only certain things can be placed into it, not all props or textures are scalable), then place a camera right in its origin (0 0 0) setting its scale upon your previous scaling (normally 1/16). I know a good tutorial. Please check:
I would suggest to simply use already made skyboxes to give the sense of a large environment or map, not caring much about watching 'real' things into it... there are many good skyboxes there
... and you hardly stare at the edge of a skybox in Portal to watch the trees or a river, right?
Now I will ask again, and maybe someone will freaking TELL ME. How the Fuck do I make a SKY!!!
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texture some brushes' sides (only those that are not flipped or angled) with the toolsskybox texture
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Go to Map > Map Properties and there select one of the available skyboxes and put its name in 'skybox' property.
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Compile and see that there is a skybox there

The 'complex way' is a 3D skybox. I can also explain how to make it but there are many tutorials out there... and there are also many threads here about the topic...
But the 2D skybox is not that hard... and it has been explained easily (though maybe you didn't find it, mate
)
You're welcome 
), but the custom textures themselves. It a hard process...
On the other hand I like to match the style of each map with the skybox, so I have many skys in my skybox folder, somes are great.
(check that link I gave you to the VDC and check how the 3D skybox must be built)
T_T will someone just post a prefab that actually works.
e.g.
```
"UnlitGeneric"
{
"$basetexture" "skybox/skyNight"
$hdrbasetexture "skybox/skyNight"
"$nofog" "1"
}
```The skybox textures are relative to the materials/skybox folder. So if you want a skybox named fractureSky: put it in materials/skybox/ and refer the skybox name as fractureSky in your skybox name field.
3D skybox
The 3D skybox is relative to 0 0 0 where your sky_camera is. Therefore my trick in order to get objects in the absolute correct position is to place objects in full size around your map at the correct position and then scale it to 1/16th of the size (0.0125) using the transformation -> scale tool (CTRL+M - press the scale radio button). Put your sky_camera at absolute 0 0 0 and scale it with your other objects you want in your 3D sky.
TADA! Perfect positions.
And Chicken, you completely lost me at $hdrbasesomethingsomething.
I don't know the first thing about custom textures and I and am not even going to go there (@_@)