It's not really that complicated of a procedure, but I'll lay it out as best I can.
First, you need to install Windows onto your Mac. In case you don't already know how to do that, a quick Google search should net you a bunch of results. Since it's free, I think that most people end up just going with a bootcamp installation. There are a ton of guides you can use, below is the one I followed.
http://www.simplehelp.net/2009/01/15...e-walkthrough/
After you have your installation of Windows up and running, follow the installation directions given on the project 1999's setup guide word for word. Link for that is below.
http://www.project1999.org/forums/showthread.php?t=2651
After that is completed the game should run fine. Most likely though you may notice issues with your character or other game elements running slow. This is most likely due to the game client freaking out about your modern multi-core processor. I ended up having to follow tips from the below thread to get that issue cleared up:
http://www.project1999.org/forums/showthread.php?t=3609