The Source engine is a bitch to build maps for compared to more recent engines like Unreal 3 or Unity. SFM maps are simpler than maps for other Valve games because you don't need to implement any game logic. Start off by opening the Source Filmmaker SDK, and running the Hammer Editor. You can find a lot of info on how to use Hammer on the Valve Developer Wiki
as well as practical information on Level design for the Source Engine
The program has quite the learning curve, so I wish you the best of luck. Let's just say that "adding HDR" will be the least of your problems.