We will add clouds and a planet.
1) Go to your assets C:\BF2_ModTools\assets\worlds
Dig around for cloud .msh and planet .msh along with there textures.
But for this tut sake:
C:\BF2_ModTools\assets\worlds\YAV\world1 has yav_sky_planet.msh and yav_sky_dome_clouds.msh. Along with there textures.
2) Copy and paste them into your word1 folder
3) Open yout Sky file
Add these these lines
Your sky file should now look like this.
Now at this stage, you should have added your BG which is coverd in this tut here: http://swbf-custom.forumactif.com/t177-adding-background
Now munge your map. And go see the results.