1 – Plug your ipod touch to the computer and open up iTunes.
2 – Hold down the POWER/SLEEP button for 2 seconds
3 – After the 2 seconds , hold down the bottom button along with the power button for 10 seconds exactly.
4 – After the ten seconds, release the power button and continue holding the bottom button until your computer makes a noise and iTunes says that your device is in recovery mode.
5 – Your device is now in DFU mode!
Now you can restore it to a custom firmware by holding down SHIFT and clicking RESTORE.
Find the firmware file and hit open. iTunes will do the rest.