So, you want to make your own skyboxes?

Here's how:
1 ----- Terragen (website has free download)
Learn Terragen and make a terrain that looks good when viewed from the middle. Set the detail to the highest. Set a square size for the skybox frames (eg 512x512).

2 ----- Run the skybox script from within Terragen (it's in the menu).
First download Skybox Script for Terragen* (right click and 'save as'). Before you use this the first time, you'll have to change something in the script. It's the part that says where to save the skybox frames. Open the script in a plain text editor. You will see:
InitAnim "C:\1-scentednectar.com\skyboxes\temp\f", 1
Change only the part I've highlighted in red to wherever you want your skybox saved.
After running the skybox script, (the script names them 1 to 6), I rename the pictures, to west, north, east, south, up, down (or left, front, right, back, up, down).

3 ----- (optional) Plugin Commander (website has free download) and free .8bf files found on the net.
Clicking on an .8bf plugin from within Plugin Commander brings up a window where you can load a skybox picture and click on the randomize button for various colour effects. When you find a nice one, open and save all 6 changed skybox frames to new names.

*I got this script from someone's website quite a few years ago. If I could remember who his name, I'd credit him here.