These are what I was told.
This guy who imports young parrots from Holland. He would spend some time with the parrots before selling them to his clients. He chose the one that come to him on cue when called. He learned this trait from the Taiwanese free fly groups. Chosing the species is also important. Best parrots to free fly are macaws, cockatoos, ekkies and conures. We don't have many choices of species here. The most popular free fly parrots are macaws and conures.
Others told me they hand fed their babies outside the house until they are fully fledged and at the same time recall trained their babies during feeding sessions. This is the best way to train them to free fly. They don't even use harness during the training.
I bought my grey at the age of 1 month as my breeder would not hand feed his babies beyond this age. I did not have the opportunity to choose the "best" and besides that I couldn't do the hand feeding outside my house as my neighbors' cats always hangout in my house compound.
In fact, I can now free fly my grey but I don't have the guts to do so. The pain of losing a grey is unbreable as I have lost one before. I had always wanted to free fly my green cheeks but none of them were interested to fly outside.