How do I jailbreak iOS 4 on my iPhone 3GS?

The problem is, I already upgrade to iOS4 with Apple's official package.

I know this isn't answering the question but I wouldn't suggest jailbreaking your phone a lot could go wrong unless you have experiance with jailbreaking. I know a few guys who have tried it and it completely messed up their phone and they couldn't bring it to apple because, well, apple doesn't support jailbreaking ha