1. Cut it into six images, starting with the panoramic portion. Name each of them 1-4. Then cut the top portion and name it 5.
2. Resize all of them to 256x256.
3. Save them as JPGs.
4. Cut the bottom portion and save it as a 4x4 JPG named 6.
5. Create a TXT called "resource."
6. Inside resource.txt type:
1
2
3
4
5
6
7. Rename resource.txt to resource.dml, if it is not a DML then you will have no skybox.
8. Put it all in a folder, name it whatever you want then place the skybox's folder into your map folder.
9. Reference the skybox in your map's MIS and you now have a working skybox.
If you need to know anything else, look at default skyboxes or other people's skyboxes. That's how I learned to make one, it's very easy.