Kiko did not trust me at all for a while after I got him. For the first week I let him remain in his cage so he could get used to his surroundings, and I sat next to him every day and just talked to him for a while. I was in my room a lot, so that was where I decided to put his cage. Also try eating and sleeping near them. Eating near them encourages them to eat, which they do with their flock, and sleeping usually encouraged Kiko to sleep. And parrots definitely don't sleep next to a predator.
You can tame them without clipping their wings, but in my opinion it's a bit more difficult. Kiko would have nothing to do with me until I clipped his wings. I recommend allowing the feathers to grow out after clipping them the one time.
After Kiko seemed comfortable around me when I sat next to his cage, I opened his cage door and would sit 10 feet away, minding my own business. I learned quickly that he wouldn't step up on anything I held, but a few times I had to force him to by toweling to administer medicine, and then I would place him on his cage. That was occasionally, though. But he realized I wasn't so bad after all, and eventually he would come out on his own. I still can't bring him out, but he will come out to visit me willingly enough if he does it by himself. And this is coming from a bird who was almost never let out of his cage.
He will step up onto his ladder he came with once he's out of the cae, and that's how I move him around. If you respect your parrots boundaries they will love you more for it, and will learn to trust you.
Apart from this, I didn't do much except for spend time with him, reading or using a tablet when he sat next to me. At first he would only eat millet I held{as long as my hand was as far away as it could be}, and gradually he learned to accept a seed or pellet from my finger.
It has been nearly 2 years and the milestones I have accomplished with him has been a large amount. But I still cannot touch him or have my fingers near him, and actually, I'm fine with that. We have a different sort of relationship.
Note~ Taming your Cockatiels might take shorter than this. Just remember to accept the boundaries they are comfortable with at first, and spend a lot of time near them. They will soon learn you are a friend, not a predator.