All birds are different. My budgie, BooBoo, didn't like head skritches and I respected his choice. But back then I didn't know about positive reinforcement training. I'm not sure if I could have changed his mind. I think I would have worked on him as I did with Ollie (my first example below) because I saw BooBoo rub his head on toys.
Ollie (Amazon) wasn't tame when I first got him and I never thought I'd be able to touch him. It took 6 months to gradually teach him that head skritches are good! I used what I saw. I saw that sometimes he would get a bell swinging and put his head underneath so it caught his head feathers. So after I had desensitized him to my presence and the presence of my hands, I unhooked the bell and swung it gently above him catching his head feathers. Over the months I made the chain shorter and shorter so that eventually I was skitching his head with the bell and all sorts of foot toys. Very, very gradually I began to include my fingers and always had a treat ready to give him afterwards. Eventually I could carefully skritch his head holding no bell or toy. Now he is always seeking them out and I use them as reinforcement for other behaviour.
Chico (another Amazon) would just try to attack me when he arrived. It took a long time to desensitize him to my presence and then when displaying relaxed body language I would briefly touch his head through the cage bars with my finger. When he was used to this, I then lightly caught a few feathers with my finger and brought them forward. I kept doing this now and again when he was receptive. Very gradually he realised what I was doing and began to enjoy it. A one second touch become 2 seconds, then 3 seconds. I always stopped WAY before he even thought about lunging. Over months he began to ask for them, and skritches got longer and longer. Then I tried through the open cage door with no bars to protect me or him. Now he also can't get enough of them. I use them as reinforcement to station at one side of the cage whilst I clean the other.
I guess in both cases I used careful desensitization and positive reinforcement, until the skritching became reinforcement in its own right.
Kobe (Pionus) and Bobbie (Amazon) are more straight forward and accepted them early on.