Create a brush.
Select one brushside (shift+Ctrl+left mouse button click in the 3D View).
Select the common/mirror texture to place it on the brushside.
Create a misc_portal_surface entity and place it in front of the mirror.
This is the map file for the tutorial:
Important: While it is allowed to place a bunch of mirrors next to each other, the engine doesn't like mirrors in front of each other. If you do it, the HOM effect (Hall Of Mirrors) will occure.
This effect also appears if you look into the void.