Getting the versions of Doom 1 and 2 included with Doom 3 BFG to work with modern Doom source ports is easy. Here's a quick guide to get the BFG editions of Doom 1 and 2 working with your favorite source port.

First, download the latest version of GZDoom at http://svn.drdteam.org/gzdoom/ or ZDoom at http://svn.drdteam.org/zdoom/. Then, unzip the file you downloaded wherever you want. After that, start the game and it should show the BFG editions of Doom 1 and 2 for play! Easy.

Before we start, I recommend getting the latest version of your source port of choice. Earlier versions do not have support for the BFG editions of Doom 1 and 2.

First, go to your base Doom 3 BFG Edition folder (C:\\Program Files (x86)\steam\steamapps\common/doom 3 bfg edition). When you're there, look for the “base” folder and open it. From there, look for “DOOM.WAD” or “DOOM2.WAD”. Copy whichever ones you want.

Now, go to the folder where your source port of choice is. After getting there, paste the files you copied into the folder that has your source port of choice. Now, start the game and it'll work just fine.

The BFG version of Doom 2 has a missing piece of code that makes it impossible to run on some source port. Most modern ports (GZDoom, Doomsday, Odamex, Zdaemon, Chocolate Doom) have ways to get around this, but with some, like (prBoom), you'll need to fix it manually.

Doing this is easy. Go to http://www.mediafire.com/?r31hb2e0ri24e7q and download the file. Unzip it.

Now, do the steps above. Now, before you start the game, drag the “d2ttlepic.wad” file to the engine .exe and drop it. Start the game, and it should work just fine.