The standard tutorial for these kind of things. Pretty advanced, actually.
Here's another one, more technical (you don't have to fully understand it). But it's good to get a feeling for how your map gets split into different areas during compiles: http://qxx.planetquake.gamespy.com/bsp/
I'm sorry, i were little busy today so i didn't add screenshots yet.
And Oprheus, the map is not going to be that big, and its dryed river so there will be no water. By the way, do you know a good tutorial for using of hint, skip and areaportal?
Nah. I copied Valves work for HL1 and improved upon it IMHO. What I meant was, the particular theme you chose will be hard to make into a playable HL2DM map.