Here's how you can jailbreak your iPhone 3GS using Cydia and Redsn0w. 1. Be sure your computer is running the latest version of iTunes. 2. Backup your iPhone 3GS to iTunes. 3. Install iOS 6 on your iPhone 3GS. 4. Download the Mac or Windows versions of Redsn0w 15b1. 5. Unzip the file. 6. Start the redsn0w app. 7. Connect your iPhone 3GS to your computer. 8. Click the Jailbreak icon in redsn0w. 9. Put your device into DFU mode (see the video).
10. Use the instructions that appear on your screen to install Cydia and jailbreak your iPhone 3GS. 11. Click on the Just Boot button in redsn0w.
For more, see the original article at the link below.
How to Jailbreak iOS 6 for iPhone 4 & iPod Touch With Cydia | Gotta Be Mobile