Summarize the following:
This is the golden rule. Many inexperienced bird owners don't realize that a lot of bird species, especially certain breeds of parrots, consider touch below the neck to be a kind of breeding ritual. As a result, frequently petting a bird around its back, wings, or tail can lead to all kinds of behavioural problems later on. While this isn't the case for every bird, they also tend to prefer being petted on the head and neck more than anything anyway, because those are the only spots they can't reach by themselves. So, it's generally better to just stay above the neck. Everyone is happier that way. This will help it to get used to you, and works as a good starting point. Be sure to be very gentle, especially when the bird is just starting to experience petting. Unlike most pets, it's generally agreed that birds prefer being petted against the natural direction of their feathers, rather than from the head towards the tail. This is good to keep in mind. You can try to gently rub the skin just behind the its beak and the sides of its head, if it still seems relaxed and comfortable. Birds also tend to enjoy being petted around their ears. (Take care around the eyes, though.) Birds also tend to enjoy being scratched under their beaks. Again, though, don't go lower than the neck if possible, in order to keep the bird comfortable. Most birds require some time to get to know and trust a person before they will allow petting or other forms of affection. However, once you've gained a bird's trust, they prove to be exceptionally loyal. Be very gentle and very patient, and it's very likely you'll get there soon enough.

Summary:
Don't pet below the bird's neck. Start by lightly touching or petting the bird's beak. Pet towards the beak, not the tail. Gradually shift your touch to the sides of the bird's head. When the bird seems relaxed and more used to petting, try petting the back of its head and neck. Be patient.