You don't want to have a map without a sky right? This may seem like a daunting task but actually takes no more than a few minutes. Making a sky benefits your map in many ways, adding mood, realism, lighting and surrounding terrain.
The First Technique
This is the easiest way to make a skybox but takes longer to compile your map.
1. When you're completely done with your map check for leaks.(see screenshot one) If you see a leak, close and or seal it off with by expanding the brush(object such as a wall) or sealing it was the nodraw tool. (if you have a displacement such as bumpy terrain as a boundary of you map it will count as a leak and you must either delete the bumpy terrain brush or seal it with a nodraw brush.)
2. Once your map is leak free go into the texture browser and type in "tools".
One of the options that will come up is "skybox" choose that and we can begin. Create a box encasing your whole map with the skybox texture. Once you have a 3d box encasing your whole map select the box with the sky brush you just made and press CTRL + H and in the textbox that comes up type in the number 12 (the default number is 32) this will make your box hollow.<-- Vital
3. The last step is choosing what you want your sky to look like. Click the map tab at the top of the screen and select "map properties..." This will bring up an "object properties" window. Click the 3rd option "skybox texture name" and type in the name of the sky you want.(look at the second screenshot) There is tons of choices (sky names) and i cannot list them all! this site can
Once you have chosen the sky name you want and entered it in the object properties window, click apply.
Be sure that you have made the skybox brush a small box that just fits around your map. The bigger the Box is, the longer the map takes to compile.
Congrats you have successfully created a skybox.
The Second Technique
This technique takes longer but is worth it because you don't need to worry about leaks.
1. Once your map is completely finished, make sure you have the 3d texture view chosen. Create a Sky Camera entity and place it in the center of your map. Make sure the Sky Camera entity is in a solid brush or another entity. This may create errors.
NOT 2. Press and hold down the CTRL key. While doing so navigate around your map and choose the outside boundaries of your map. (Holding CTRL and selecting multiple objects will select them all instead of selecting them one by one.) As I said before move around your map holding CTRL and select the outside boundaries of you map. They must be BRUSHES not OBJECTS/PROPS or ENTITIES. The boundaries are the highest brush on your map, the lowest, farthest north,south,east and west. Select your sky camera as well (or your skybox will not work!!!)
3. once you have selected your sky camera and your outside boundaries you may let go of the CTRL key, right click in one of your building views and select copy.
4. Then paste your copied crushes and sky camera outside of your map. With your newly pasted brushes outside you map still selected click the Tools tab at the top of your screen and select transform. A small window labeled Transformation will come up. (see screenshot 4) Select the "scale" button and type in .0625 as all the values (X,Y,Z) Then click ok. What this does is shrink your newly pasted brushes and sky camera to 1/16 its size. Place it outside your map.
5. ALMOST DONE. Now with your mini brushes and sky camera outside your map, choose the skybox texture and create a hollow box around your mini-brushes and sky camera that are outside of your map. Make sure its hollow by selecting it and pressing CTRL + H, then typing in 12 in the window that comes up. (see screenshot 5 and 6)
6. One more mall step. The last step is choosing what you want your sky to look like. Click the map tab at the top of the screen and select "map properties..." This will bring up an "object properties" window. Click the 3rd option "skybox texture name" and type in the name of the sky you want.(look at the second screenshot) There is tons of choices (sky names) and i cannot list them all. But this site can Don't forget to put a copy of your light_environment entity in there too.
Once you have chosen the sky name you want and entered it in the object properties window, click apply. Also go back to your actual map (not the one surrounding the skycamera for the skybox) and delete the original skybox entity.
And once again you have successfully created a skybox. Texture Shrinking
You may notice that the textures in your 3d skybox are significantly larger than the ones in your actual map. That's because the sky_camera enlarges the 3d skybox and its textures by like 16x or something like that. To counter this you must Select all the textures you want shrunk in the skybox with the texture application tool. In the Texture Application Tool window, there should be Texture Scale. Set X and Y to 0.05.