• Save each map to the "C:\program files\steam\steamapps\[steamid]\half-life 2 episode two\ep2\maps" folder
• Unzip any that are zipped
• Run  Half-Life 2: Episode 2
• Enable the developers console. (it's off by default) You can do so through Settings -> Keyboard -> Advanced -> Enable Developer Console.
• Open the Developer Console by pressing ~ or ` (the key beside 1 and under escape)
• Finally type "map " (without quotes, and a space) then the name of the map, and press enter.

  For example: map TGorang_GMLair_v09

Assuming you have Half-Life 2: Ep 2,

Practice with this Map: TGorang_GMLair_v09