You need to check your bootrom first. If it is the new bootrom then it will not work. There are tutorials on how to check your bootrom for windows and mac if you just google.
sorry I missed that you aren't activated. That won't work.
You should be able to jailbreak using Spirit and then use Ultrasn0w to unlock if you just want a working phone. There is currently no way to get iOS 4.0 jailbroken on the new bootrom though and by using Spirit you might be closing some doors for future options through PwnageTool.
Google "downgrade 3.1.3 to 3.1.2" then in the first hit look for the link that says tutorial and it will explain the steps necessary.
If you get an Apple logo recovery loop you can try iRecovery to get it out of this loop.
Yes you can downgrade to 3.1.3 and jailbreak using Spirit. Then use Ultrasn0w to unlock. You just won't be able to go to a jailbroken 4.0 after using Spirit to jailbreak 3.1.3.
You have to find a copy of the firmware and install it through iTunes. If it is on file with Cydia you will have to do some work to get iTunes to verify the signature on Cydia's server rather than Apple's server. There are plenty of tutorials out there for that.
That should work if you have a trusted source for the custom firmware. Just make sure you are in recovery mode when you install it.
If you have SHSH on file you should be able to downgrade. Once downgraded you will have to jailbreak using something other than Spirit. Then use PwnageTool 4.01 to create a custom firmware and install it through iTunes with the iPhone in recovery mode.
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