1. Start by going to marathon.sourceforge.net. You’ll find all the files you need there.
2. Scroll down to the Downloads section and click the links for Marathon, Marathon 2: Durandal, and Marathon Infinity to start downloading the data files for all three games. Go ahead and unzip each game directory wherever you please.
3. Now that you have the data files for each game, you’ll need to download the newest version of the executable that so you can play the games on your modern PC. Scroll down a little further to the Engine section, and click Download for Windows. You’ll download a file called ‘AlephOne-20110731’. (The date listed in the file name may be different–it indicates the build version of Aleph One that you’re using.) Aleph One is the name of the open-source version of the Marathon engine that you’ll be using to play each game.
4. Unzip the contents of the Aleph One directory wherever you prefer.
5. Right-click on the AlephOne.exe file, and choose Copy.
6. Open your M1A1, Marathon 2, and Marathon Infinity directories. In each one, right-click the folder window and choose Paste to copy AlephOne.exe into every directory. Make sure that you’re in the same directory as the data files–‘Map.sceA’, ‘Music.ogg’, and so forth.
7. Now you’re done installing Aleph One for each Marathon game. When you wish to play, just open the AlephOne.exe application in the directory of the game you want, and it will open.