Download the 8 cfg files and place them in .../Portal2/portal2/cfg (for me it's C:/Program Files (x86)/Steam/steamapps/common/Portal2/portal2/cfg)
The a_light file binds your numrow to change ambient light so if you don't want that, delete it
When in Portal 2 type in "exec a_reset" in the console (this only needs to be done once)
Type in the number of the level on the numpad (for example to go to 2-3 Team Retrieval you type in 1 then 2 on the numpad)