First things first. In order to play, you will need a fully licensed copy of Quake 2 for Windows. If you don’t have it, get it. It’s pretty cheap. This article is just for getting Quake 2 set up for BeOS users, so from here on out, I’m assuming that we all own the Quake 2 CD.
Download Quake 2 for BeOS and install it into /boot/apps. If you choose another location, please note where you put it.
When you open your Quake2 directory, there is a README file that gives instructions on how to import the game data from your CD. Ignore this. It doesn’t really work too well. We’re going to do it manually.
First, put your Quake 2 for Windows CD into your Be machine and mount it on the desktop (right-click the Desktop, select "Mount" and look for the CD). Open the CD and navigate to /INSTALL/DATA/BASE2Q and find the file PAK0.PAK. Copy this file to /boot/apps/Quake2/baseq2. Rename PAK0.PAK to pak0.pak. BeOS requires that this be lowercase.
This same problem applies to the baseq2/players directory. You need to rename EVERYTHING in this directory to all lower case. But, the good news is, someone has done this for you! Download this version by Sarev and unzip it to /boot/apps/Quake2/baseq2/players .
Lock and load, folks! You’re ready to Quake! If you’re going to use OpenGL, make sure your video card supports the BeOS hardware OpenGL implementation. If it doesn’t, you’ll crash, have to reboot, and set video rendering back to "Software.". In other words, you probably don’t want to have 40 pages of unsaved word processing docs open when you launch Quake 2 for the first time.
See you in the Deathmatch arenas — watch your backs!

Anonymous adds:
You don’t have to use Sarev’s patch. If you want to do it manually, just open copyq2files.sh in a text editor and look for the if statement checking for /quake2. Change this Quake2 (capitalized), save, and run the script again.