You need a terrain editor like
Terragen.
Once you have created your terrain, then you place your camera in the center. (absolute center or it won't work right)
Then, you aim your camera in 90 degree intervals saving pictures of the rendering of the 4 sides around you.
Then, you tilt it up 90 degrees and down 90 (180 from your up position) for the top and bottom shots.
You need to have the settings right for the camera for it to end up seamless.
I used to know of a great tutorial for doing this, but now I can't find it. (maybe it was taken down by author or something)
Bryce is another popular terrain editor, but I've always used Terragen.
Edit
Here is a
tutorial for Terragen that includes detailed instructions for the camera settings.
You can skip the last part about importing the skybox because you will be using a different method for FPSC.
I made a free skybox tool (ceSky) that is in the tool list, which packs the 6 images that you created for an FPSC skybox.
The instructions should be there with the download, but it is really easy to use.
You just import the 6 images, then you can preview it, then save when you have it right.
It will convert your images to .DDS files and place them in an FPSC compatible folder, so you copy the folder and place it in your skybank folder.
Sometimes you will need to rotate the top and bottom images 180 degrees if you were thinking front is the direction towards the user in terragen.
Play with it and you will probably get the hang of it fast.
