Let's make a sky (skybox) with SkyPaint and Quake Army Knife editor

 A skybox is a texture in a closed space with up, down, right and left coordinates (your map or bsp in Quake Army Knife, bsp in Quake 2 or DoD).
After you open SkyPaint program (links on the end of this topic), you have the option to modify the preferences or if you want the default sky or just save the default scene as tga and pcx (more about Quake Army Knife editor usage later).

SkyPaint will have a doc offline file from the program, if you click the book icon.
When you install SkyPaint, .8bf files will be in your folder. In the plugin path put their path, in Paint Program, i put Paint Shop Pro 7 path.

 TGA and pcx files are saved from SkyPaint. You can close SkyPaint for now.

From Paint Shop Pro 7, go to File-Preferences-File locations-Plug-in Filters. In the box, put your SkyPaint path.
Now, when you make a image in Paint Shop Pro 7 and click Effects, you will see Plug-in Filters-SkyPaint.

 Open SkyPaint, click paint this view, 256 and 256, and it will open Paint Shop Pro 7. SkyPaint will close by itself.

 The Plug-in Filter, from where you can move left, right, up and down in skybox ,,frames,,

Do something and from The Plug-in Filter, launch SkyPaint again.

Save you image. It will save in multiple images (TGA and pcx) with abbreviations for up, down etc.

Now you can use your Quake Army Knife editor and a program that opens a .pak file (a archive type file with folders inside that have sounds, maps etc.) I used PakScape.

Copy and paste a existing pak file that has env folder in it.

In pakscape, delete all folders and insert only the env folder with only your pcx and tga files, which are images for sky parts, made with SkyPaint.

I saved the work in a pak4 file (before was a different pak3 file used for something else).

In Quake Army Knife editor, double click worldspawn and write the word which represent your sky, in my case green, don't write green_ or greenup.

Double click sky

Click the box pointed by the green arrow in the image and write 12000, representing the light intensity. More may be neccesay, but it is up to you.

In the game, the sky will show, but the room you are in will be black, if you don't put some light in it
Yes, the pointy thing is the knife :)

After you followed the above tutorial, read this link to make your sky have shadows on the ground :
Quark sun light tutorial


More info about skyboxes :
gamasutra skybox
SkyPaint program :
skypaint
SkyPaint program download :
skypaint download
More info and programs :
developer.valvesoftware
PakScape :
Is used to import/export/delete files, folders from the opened pak file, which is some sort of a archive. You can rename the .pak file to .rar and vice versa but it cannot always work. Quake Army Knife can open pak files, import files, but cannot make folders easily.
PakScape