Some of the textures in HL2 can be partially reflective. How do you do this? inspecting the materials room floor in sf_fieldtrip, I placed a cubemap above the floor that I wanted to be reflective, but nothing happened. I thought that maybe the texture I was using didn't have any reflectiveness data or something, so I used the concrete texture from said materials room floor, and it still didn't work. Can someone help me, or better yet, write a tutorial on how to do it?

Well at least you have to go to env_cubemap's properties and there somewhere reads pick button. Click it and then select all surfaces where you want that env_cubemap to effect.

what about buildcubemaps in the console?

There is no need to manually pick the surface within cubemap properties as long as it is within it's general vacinity. But once the game starts up, in order to get any effect from the cubemaps, you need to open the console and enter "buildcubemaps". This allows the cubemaps to run their calculations using the fully compiled and rendered map (which could not be done within hammer).