For the link-impaired:
Now, follow our instructions below to the letter and you?ll be firing your gravity gun through portals at unsuspecting Combine in no time.
* First off, backup up your entire Portal directory in a safe place. It?s the folder marked ?portal? in ..\Steam\steamapps\(your_username)\. We, nor Valve, take any responsibility in the unlikely event of you hosing your game files. Worse comes to worse though, you could always just redownload.
* Next, download and install, if you haven?t already, GCFScape. This handy program will allow you to access the cached game files for any Source-powered game, in addition to several other Steam games you may have downloaded.
* Once that?s done, browse to the directory ..\Steam\steamapps\. There, you?ll see all the available GCF archives. We?ll start with Half-Life 2.
* Open the GCF marked ?half-life 2 content.gcf? You?ll see a directory tree in the left pane. Click the box to the left of the folder marked ?hl2? to see the directory listing contained within it. Then, right-click ?maps,? select ?Extract? and extract the entire folder into the directory \Steam\steamapps\the_great_bundini\portal\portal. This will merge it with the existing Portal map folder.
* Do the same with the ?scenes? folder
* Fire up Steam and run Portal. When it?s done doing its thing, open up the developer console with ?~? (you may need to enable it under your keyboard options).
* At the console, type ?maps *? to list all the available maps. Now that you?ve imported the Half-Life 2 maps, this will be a fairly lengthy list and some of the names, like citadel and canals, should seem familiar.
* Type ?map? and then the map name to open that map. For instance, ?map d1_canals_01?
* When the map is done loading, pull up the console again and enable cheats by entering ?sv_cheats 1?
* To give yourself all weapons, including the portal gun, enter ?impulse 101?
* Depending on how far you got in Portal, you may need to upgrade the portal gun to fire both portals. Enter ?upgrade_portalgun? to do so
* Go lay waste to some alien scum with the power of portals!