Get the form off the CASA website, fill it in, pay the fee, wait for them to send you the new bit of paper for your licence. Simple as that!
Ah, not quite. Did my CIR renewal last October and did exactly that to try to get the PIFR. Got a letter back from CASA kindly returning my form and advising me to get the ATO who did the renewal to sign off on it. Still waiting for him to get back to me....
Moral of the story, I guess is to have the PIFR form ready for ATO to complete along with CIR renewal paperwork...