I have started to make maps in on Source SDK, I think I am doing alright for only playing around with it for 6 hours. This is what I have learnt:
Making skybox
Making houses
Making rivers/oceans/lakes
Adding dynamic lights
Making scenery
Blending the scenery
Adding static props to the maps
Adding hills/mountains
Add doors
I have a test world that I am playing around with that I have completely built up from nothing.