I hate to use the word WILL, so will opt for SHOULD.
On completion of your 15th cruise, you should automatically become Elite members with all the associated benefits. You should also receive your pin whilst on board.
If you are still on board, then go & check with the loyalty manager who will be able to tell you exactly what the 'current' rules - just in case they've changed over the year ;o)
Well done on having 15 Princess cruise memories.
Last: Oceania Nautica Aug 2016
Next: Cruise Maritime Magellan Nov 2016
Then: P&O Oceana December 2016
Golden, Grand(2), Caribbean, Royal, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(3), Oriana(12), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(7), QE, Caronia(2),
Oceania Nautica, Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey