Hi Lynnr,
I would suggest the same as Tari, if you have the Aviator Harness, it comes with a DVD that very clearly demonstrates how to get you Parrot familiar with the harness, touch training to get him used to you touching him under his wings, spreading his wings etc and how to slowly go through the methods of putting on and taking off the harness.
It is not natural for them to have a harness on, even if they are young. It is more difficult though with an older bird.
Good luck!! :-)