Harness training. How well I know ye. Salty and I worked EVERY NIGHT for almost a whole year before he was comfortable enough to wear one and not freak out. First step was yes, let him be comfortable around the harness. Next, using his abso'ute favorite treat, to entice him to put his head thru the loop to get the treat, progressing further and further thru the loop, until one day it was all the way thru, and I draped the rest of the harness over his wings and body. Weeks and weeks of just that. Always use the same ON command, and the same Off command. And slowly made the wing loops larger and larger, until they were able to be slid under his wings. And then one magical day, when he had it on and under his wings, I was able to slowly tighten up the body loops. As I said, a year, 365 times at least, but Salty will wear it now no problems, and does not spend the whole time trying to bite thru it. I started the process when he was maybe 4 or 5 month old, he will be 2yrs old next week.
Be prepared to take steps back sometimes, but try for little bitty incremental steps forward. And I'm will never free fly him, I have read too many stories of parrots that were trained by the best , only to have been scared by something, or a sudden gust of wind, and never seeing their loved pet again. Good luck.