Whilst containing details for using an alternative sim from giffgaff (02 network piggyback) I think this video will show the underlying process. Says he who hasn`t even got his Android phone yet and not sure if I`ll end up with the San Fran II depending on stock levels and delivery from Orange or the Monte Carlo (in shop up to now).
Unlock Orange Monte Carlo (ZTE Skate) free - The giffgaff community
Have a look and see anyway, make sure it matches the text instructions you read from here too. It looks the same but I`m no expert. Hope this helps to give a little more confidence, it has me.