Let it restore and eventually you will get a "Unknown error occurred" (1015). Now you have a 1.1.1 iPhone with the 1.1.2 modem firmware.

Now you need to enter DFU restore mode.Hold home and sleep buttons and when the screen turns black let go of sleep and keep holding home. iTunes will recognise this and you will hold shift and click restore and navigate to 1.0.2 firmware files.

Let it restore and you will get a "Unknown error occurred" (1013) and the yellow triangle should pop up. The hard bit is over - you're nearly there!

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