To create a skybox, set aside some part of the level as the skybox. Place a
thing of type 9080 inside the area you created to signal that this is a
skybox. The position of the thing is the position the sky will be seen
from. The first argument to this thing specifies how visibility for the
skybox. Skybox visibility is controlled independantly of the rest of the
map (which is controlled by the cvar r_visibility). The visibility used
for this thing is four times the corresponding value of r_visibility. So
to use the standard visibility of 8, set the first argument to 32.

Leave the 9080 thing's TID as 0 to make it the default sky for the level.
If you give it a non-zero TID, then you also need to use a 9081 thing in
any sectors that you want to use that skybox. The 9081 thing's first
argument should match the TID of the desired 9080 thing.

If you use a 9081 thing with a first argument of 0, then its sector will
use the standard sky texture instead of a skybox, even if you set a
default skybox for the map.

If you have noisy things inside the skybox (such as projectiles), you can
place a 9082 in the skybox's sectors to prevent things from making noise.

The file thebox.zip contains an example wad that mixes two skyboxes and
the standard Doom sky.