Place env_cubemap entities in valid locations in your map, and compile. Important: Name has to shorter than 30 characters (including the .bsp extension), or the .bsp won't be writeable! Also, don't change the map name after compilling, or the cubemaps won't work!
Use your favorite .bsp pakfile editor (For example, VIDE or Pakrat. Usage of these tools isn't included in this tutorial.), and load your .bsp.
Delete all files that are a "Texture" or "Texture (HDR) type. (Not "Material" files, only the .vtf's!)
Save the edited .bsp.
Copy your map back to your game's maps folder.
Open console and type: "map -mapname-". (In my case, "ttt_mars_colony_cubemap".)
Once you're ingame, open the console again and type this: "mat_specular 0" and "buildcubemaps".
Wait until the cubemaps are built, and then re-enable specular mapping with "mat_specular 1" and reload your materials with "mat_reloadallmaterials".
About the map itself, I think it looks pretty good, but some parts seems to be a bit too dark... But overall, it's a pretty good job !!
Note : If you want to pack custom content with your map, do it after building cubemaps.