Simple example of the triangle method of doing floors and hills for half life.
This example is for user 12114.
This is a piece of an old map so I am unsure of the textures. Re-texture!
Included is a updated .vmf version, also simple, but uses updated textures.
Ultimately, use displacement surfaces for half-life 2.
also, its hard to texture terrain done this way correctly. setup the texturing when it is still flat THEN deform it, or you'll learn it the hardway.
This is why I don't map for half-life 1 anymore. I admit that steam is a major pain but it is far better for me to use the half-life 2 (source) than gold source or half life 1.
it's an example, get it?