How do you import custom terrains?

I have seen other maps that have custom terrains and well, I want to make a map with a custom terrain too :> . I just don't understand how to import it into the map and being able to use it . I already tried to import it but I dont know how to use it...

you need to import it over certain terrain. Say you dont want the cityscape terrain import your new terrain over the city scape terrain.