The custom_sky_tutorial.zip file you have requested: Tools and Utilities / Tutorials / Custom Sky Tutorial - 344.49kb is not available. Perhaps try browsing all files for the game Halo: Combat Evolved and upload any files you want to share or are missing. This is the only information we could locate:
This is a PDF describing the process of how to make your own custom sky for Halo Custom Edition. It does go step by step and provides pictures as well. It is a good download in conjuction with FD04's Outdoor tutorial.