How to jailbreak IPHONE 3G Firmware 3.1.3 (5.12.01) NOT 5.11.07?

the title says it all, i do not want to UNLOCK it i just want to jailbreak it, no youtube videos are helpful, a lot of them are titled 'jailbreak iphone 3g 3.1.3' but then use an ipod touch or they use a 2G iphone. and the rest of the videos are telling me how to unlock after its jailbroken, and Redsn0w did not work with my iphone. Also if someone knows how to force downgrade from 3.1.3 (5.12.01) to 3.1.2 (5.11.07) it would be great, i tried to download the 3.1.2 firmware and restore it but it says its not compatible. thank you!!