You can either complete the form that P&O send you and then return it or a quicker way is to complete it on-line. Go to the P&O website and then click on Before you sail. Then you will see Cruise Personaliser. Click on that then you enter your name and your P&O booking reference no which is on your booking form. Then you will see all the details of your booking and you can complete the necessary details there.
We have always done this online, once completed it states this when finished, brill.