You could (a.) use the 6pic method where you string together pics of a landscape made in terragen, 3dsmax ect, or (b.) you could use a props_skybox entity (I think thats what its called…) and build your terrain with brushes/patches or you could model it and import it as an md3 or ase or whatever you want to use. (Personally I perfer to option b)
Im at work now so I cant post up a bunch of links but I will when I get home, if no one else has by then.
