Custom Skybox Texture

I wanted to create a simple space texture and a couple other skies but don't actually know how. Can someone please tell me? Thanks

http://fps.brainerd.net/terragen.htm

It's for Quake3, but it definitely applies to any FPS that has skyboxes.

Ignore the stuff about the shaders and what not, and just pay attention to the main tutorial.

Here's a very similar one:

http://www.delphigl.de/tutorials/skybox.html

Ok, Allow me to rephrase my question. I would like to IMPLEMENT custom skies in my map. How do you take bsps, or tgas or whatever it is, and use it as the sky in your level? I know u have to take 6 different pictures and make a cube or something like that, but I don't understand how to get it into your map.

Thanks blitz for the program, but what I need is to know how to use the skys i've created.