It's not really so hard to create custom textures for Doom 3, as long as you have pictures or something, here's one easy way to add custom textures to this game.
If you wan't to have outdoor places in your maps, you will need some kind of skybox to make it look good, I will make a tutorial on creating a custom skybox later on.
Make hills and other terrain easy as pie, the terrain I make in this tutorial looks like shit though, but just keep on tweaking it and it should hopefully look really good.
This will show you how to make a mirror, it's really just a texture but still, wanted to show anyway. *HINT* Don't make a mirror which could see/reflect another mirror.
Decorating maps can be pretty difficult if you don\'t want to use the default model props, here\'s a pretty simple way to create a handrail which is nice for most levels.
Need to access high places faster? This is what I recommend for multi-player maps with Doom 3, you should also trigger them to a SPEAKER with an elevator sound.
Need to access high places? Ladders are one of the ways to do this, however I find them most useful for single-player maps with Doom 3 since climbing is pretty slow.