To load custom maps into Half-Life 2, you would need to download the map, of course. Good places to download these maps are places like GAMEBANANA.com and Garrysmod.com (if the map only has Half-Life 2 content in it). Once the map is downloaded, copy it to this folder C:\Program Files\Steam\Steamapps\common\half-life2\hl2\maps. This is the location for the maps if you have already "Steam-Piped" Half-Life 2 or any of the episodes.

Then, start the game and press the tilde key (~) to open up the developer console. If the console does not come up, go to your options and go to the keybindings tab, and press the "Advanced" button. A small window opens up and check the box that says "Enable Developer console". Now you will be able to open the console.


Once you open the consiole, type in "map" and then put in the file name of the map after that (excluding the .bsp extension).

There you have it, now you can play any custom map for any specified Source game.