http://gamemakerblog.com/2013/11/25/gamemaker-studio-standard-edition-is-currently-free-50-value/

Download, install, and let it update.

Edit: A poster mentioned you need to select the “Beta” update option (you’ll see it) to get the option for the free license. That’s the option I picked.

The program will restart and launch with a window with three options: one to use the *free* (feature-gimped) version, one to register for a free studio license, and one to enter a studio license.

It’s the second option. Enter your email. Enter it again, hit register. Key comes to you. Enter the key. VOILA.