Adding custom textures?

There's not much documentation on Doom 3 things, I was wondering what are all the necessary things to properly have a custom texture in the game for use with the map editor.

Doom 3 seems to require a specific DDS format because every time I try to make a DDS texture it fails to load.

Here's good documentation about Doom 3 texture handling:

https://www.iddevnet.com/doom3/materials.php

https://modwiki.xnet.fi/Texturing

Basically you need to have your images and learn about Materials in order to have proper custom textures in Doom 3.

Thanks, I'll give it a shot.

I think, this tool, despite of age, can prove useful for .dds. It's already is mentioned in Arl's link, so, I might just made an emphasis on it's engagement.
http://wiki.thedarkmod.com/index.php?title=DDS_creation.

The .dds files are only used in the lower quality settings. Just use .tga files for your custom textures and you are fine.

For a regular texture in Doom 3 you need a material file (text file renamed to .mtr) and 3 textures: diffuse map, normal map and specular map.