I think I read a tutorial on it somewhere, but can't remember where exactly. Anyway, it's pretty simple.
Make a window, put the glass texture on the side you want to be opaque, and put the NULL texture on the other, so you won't see anything there when you look at it from that side. You don't make this transparent or anything, but it's probably a good idea to make it a func_wall.
You then make a second window up against the first one on the side with the NULL texture on it, and make this one like a normal window with transparency.
I'm not too great at tutorials, but I hope it's easy enough to understand.