You will need to know your firmware to be able to get a correct, down gradable firmware in some cases. it is recommended that you know it anyway.

http://www.felixbruns.de/iPod/firmware/

Put the .ipsw file somewhere easily accessible, like on your desktop.

You need to save your SHSH blobs to be able to successfully downgrade without problems.

You can get a program called TinyUmbrella to save the shsh blobs. It is at: http://thefirmwareumbrella.blogspot.com/. Using it is very simple. Just connect your device and press "Save SHSH".

THIS IS CRITICAL!

You must put your device into DFU mode to downgrade, do this first by going to the "slide to unlock screen", then holding down the power button for 3 seconds hold both home and power for another 10 seconds, then let go of the power and hold home for 15 seconds.

Open up iTunes, then iTunes will say that your iPod is in recovery mode.

This is the point of no return! and after this,you assume all risk!(no risk before and I almost screwed my iPod up here!)

You will need to go into TinyUmbrella, and start the TSS Server, to stop Apple knowing you are downgrading.

Assuming you downloaded the .ipsw before, hold down the option key and click restore, find your restore file (should be on the desktop, then choose.