Confused re: when to turn roaming on

I have the One plan and have added on the 30 day international plan.  I was told to turn on Roaming and then once I turn on my phone in Italy, I can make calls/texts, etc.  

When should I turn on roaming - at the the US airport before I actually leave for Italy and then turn on airport mode? OR turn on roaming once I arrive in Italy and after I turn on airport mode?  I'm so confused.