You don't need to be an O2 customer, and your device doesn't need to have been bought from O2 to be eligible.
Step 2 - Get a quote
O2 will value your device, and you can decide how you'd like to be paid. The options are bank transfer, cheque, credit on your O2 bill or an Amazon.co.uk gift voucher (which is usually of slightly higher value).
If you're happy with the quote, and your phone doesn't have any obvious damage, just click the "sell my mobile phone" button.
If your phone is damaged, you should select the "give you a more accurate quote" link, which will prompt you to answer three questions about the condition of your device. It's possible that this will result in a lower quote, but this will aoid any disappointment further down the line if you hadn't disclosed it.
Step 3 - Log in and confirm
If you already have an O2 account just log in with this, otherwise you will need to create a new one. Confirm that you want to go ahead and O2 will let you know what you need to do next and where to send your gadget.