Bystef, It took me awhole year, of gradually training my Amazon Salty to accept and take off his harness. Every single day, that was 5 yrs ago and we still pracitice it at least 3-4 times a week , just putting it on and taking it off. Like any "trick" you break it down into small steps , be consistent and reward immediately when the step is done correctly. I would wait awhile before commencing it again, regaining your parrots trust is more important.
You need to be able to handle, manually, your parrot, to manipulate the harness and his wings ,his head an the neck loop, with him being OK with that. It takes time usually.
Check this out....
https://m.youtube.com/watch?v=iw16lN2T0ik
Right about here........................................................................................................................
^^ I get it right.