Back in the days when Doom 3™ was new and shiny Josh ‘b0rg’ Holmes, gaming editor at VIAVGA, created a fantastic guide to getting started with mapping. Its focus was multiplayer, but it’s just as relevant to single player.
Sadly, the VIAVGA website seems to have gone, but we archived the content and created a handy PDF with the whole guide in. Download here: Doom 3™ Multiplayer Level Editing Guide. Aplogies for the quality of the screenshots but I think they may have been that way in the original article.