How do I add more materials?

Hey guys. So I recently been building a ‘house’ and I wanted to make a variety of wallpapers and etc.
But I noticed I have not much materials.
I downloaded custom materials but they didn’t work (did not add to materials tool).
I have CSS, HL2EP2, Portal, HL2DM, HL2 Lost Coast.
So, how do I get more materials? How do I apply custom materials so they can be seen in the material change tool?