I've seen many people just put the harness on and give lots of treats and take them outside right away. And now the birds love the harness because they know they get to go outside. What is wrong with this method? Many people do this.
What is wrong is that if it doesn't work it is hard to recover from. I tried the "flood them and let them see how cool outside is" method with a Brand X harness. Corsair
spent the whole trip outside fighting the harness. She hardly noticed where we were. Then I found The Parrot Wizard site, bought an Aviator harness and went back to desensitizing--which took longer now because of the previous bad experience(different design and color did NOT fool Corsair). I would hang the harness on her training perch and she would pick it up gingerly and drop it to the floor(it WAS funny to watch). We worked through that, and moved up to it touching her, putting her head thru a body loop to get a treat, putting her head thru the head loop, and, many months after the first attempt, wearing the harness for less than half a second. She still isn't happy, but she doesn't leave or bite. We are working on wearing it longer now.
You might be luckier, but I wish I had known to do it right the first time.