Update - Iphone 3GS 3.1.2 (jb-ed) with new bootrom to Custom Firmware

[narutiment]

Hi guys,

Currently I have a iPhone 3GS:
-3.1.2 jb with blackrain
-new bootrom, with SHSH 3.1.3 onwards up to 4.2.1

I would like to upgrade to untethered jb, hopefully with Custom Firmware. Any version is fine with me as long as it is stable and does not lag when operating.

And will it be able to downgrade back to 3.1.3 or 4.0 in the future?

How do I go about doing it?

Thanks!!

[Cowboy]

Use google no one can post custom ispw on here lol And just use redsn0w