UNZIP IT IN;
C:\Program Files\Valve\Steam\SteamApps\your_account_name\half-life 2\hl2\maps
Start Half Life2 and open the console by pressing;
type in the console;
(the maps are quit big so it could take a little while before they start. and when you have difficulty playing them,((it starts to look more like a slideshow)) because you don't have a very powerful computer than you could try to lower the graphics settings a bit in the menu.