Holtz cautions that purplera1n is in beta and recommends a full backup of the phone is made before running the software, but the procedure seems pretty straightforward. As detailed on his blog: “Connect your iPhone normally. Click ‘make it ra1n.’ Wait. On bootup, run Freeze, the purplera1n installer app.” This will allow iPhone 3GS users to install any application directly onto their device, including those not sanctioned by Apple, but it won’t free you from your current carrier. The ultrasn0w tool should take care of that.
You can find a download link for purplera1n here and additional details on Holtz's blog, including some crash reports on the comments that you’ll probably want to sift through before taking the plunge.