To port your number in you simply need to fill out the 'Transfer your number' form (you'll need to be logged in and already have activated your SIM).
You'll need the following info:
The mobile number you want to keep
The PAC code (Port Authorisation Code)
Your preferred transfer date (note: you can't transfer your number over the weekend or on a Bank Holiday. If you do choose one of these dates the system will select the next available day for your transfer). The date chosen must also be within the next 14 days.
An alternative contact number (just in case we need to get hold of you)
The transfer form can be found herehttp://giffgaff.com/support/transfer