Firstly, Ensure that you have an active goodybag or airtime credit available on your account
From your handset:
*Dial *100*7# to check your goodybag allowance
*Dial *100# to check your remaining airtime credit balance
If you've not already done so, You will need to install the (APN) settings into your phone, If your unable to create your own APN and save this as the default one, Try and edit an O2UK APN with the required giffgaff APN settings, Select and save, The reboot your phone, Once you see a 3G/H symbol, Your completed.