If you have downloaded a mod ( .pak file) or are setting up a source port, follow these steps:
Creating your own PAK file is straightforward. Using a tool like PakExplorer , you can simply create a new archive and add the folders and files you've created (like a new maps folder with custom .bsp files) into it. Save it as pak2.pak , place it in your id1 folder, and your custom content will be loaded by the game.
: This file contains the remaining episodes and content. Downloading this file for free from third-party sites is considered illegal piracy
PAK files do not contain the soundtrack. The original music was played directly from the CD-ROM. For modern play, you'll need to download music tracks separately in OGG or MP3 format and place them in a folder named music inside your id1 directory.
If you are a content creator or a heavy mod user, you might need to look inside the PAK files.