I talked a bit about how I taught Boomerang to each of my parrots on my
Parrot Wizard Seminar DVDs. I'm gonna leave it at that cause I think it's a tougher trick that out of the right context will make people create problems with their parrot.
As for "freeflight" it's something the birds choose to engage in. I would say the best thing you can do is to teach your parrot to recall and boomerang to you. Then in a large enough of a space, instinct and curiosity might take over and the bird goes and flies around. The first time it happens your heart will sink and you will panic. But then you'll realize your bird has it under control, is doing it on purpose, and is even enjoying it. I realized this for the first time when I was flying Kili in the school theater. There are numerous articles and videos about that. If you go through those (I highly recommend seeing all of them to see the progress), I think there is footage of her just flying around. As long as that "flying around" culminates with coming to me, it's all good!
My birds are so well trained that they stay on their perches until called and wouldn't just go freefly on their own. However, if I give them permission, they go and fly around and come back to me when I call or they feel like it. Basically I just toss them off my hand into the abyss saying "go fly" and they go enjoy themselves for a bit.