My experiences with cockatiels have been that their attitude is at least partly proportional to the length of their flight feathers. Here is how I do intensive taming with a frightened or aggressive bird.

Step 1: Catch the bird in a towel (2 people for this would be good) and clip its wings fairly severely. Put it back in the cage and let it calm down/recover for 24 hours.

Step 2: Open the cage and let the bird walk around on/outside the cage if he will come out. If not, stay with step 2 by leaving the cage door open for an hour or two each day until he is brave enough to emerge. After 4 days go on to step 3 anyway. Have a perch-sized stick handy - if the bird jumps or falls off the cage, get him to step onto the stick and return him to the cage.

Step 3: Approach the cage with the stick (a pencil works for a cockatiel) and try to get the bird to step up onto it, then off, repeatedly. When he will readily get up onto the stick, move to step 4. If the bird will not get on the stick from the cage, towel or catch him gently and and take him to a small, relatively empty room. (Bathroom, closet, utility room type). Close the door, place him on the floor, and sit down near him. Work with him on stepping onto the stick from the floor until he loses his fear of the stick. Then go back to the cage and complete step 3.

Step 4: With the bird on the stick perch at about waist level, try to get him to step onto your other forefinger. If he doesn't startle at the sound of your voice, start saying Up and Down. If he falls or jumps, pick him up with the stick and try again. When he will readily step onto your finger from the stick, start getting him off the cage with your finger. When he will reliably step on to your finger from the cage, go to step 5.

Step 5: Hopefully by now you know what one or two of the bird's favorite treats are. Offer one on your palm, or two fingers, so that he only has to reach out to get it. When he is comfortable with that, move the treat so he has to step onto your hand to get it. Then offer it between thumb and forefinger. When he will calmly and easily take the treat from your fingers or hand, go to Step 6.

Step 6: With the bird on your hand or finger, offer a treat. While he is eating it, gently reach up and touch the breast feathers. If he doesn't jump away, stroke the breast gently for a second or two, then offer another treat. Alternate stroking and treats until he doesn't pull away from the touch any more. Then slowly progress up the chest to the neck, around to the cheeks, and to the top of the head. Some birds will eventually let you go under the wings, too, some never will.

When step 6 is completed, you have a fairly calm, tame bird that is not really afraid of you any more. From here on, work on your own towards the things that you want the bird to do, such as talking, simple tricks, cuddling, etc. At any time if the bird gets overly stressed or freaks out, go back to the previous step for a day or two, then try to move on again.

Yes, this is a long, time-consuming, frustrating process, but it generally works and minimizes stress to the bird. I have successfully tamed/re-tamed wild-caught and never tamed, hand-fed then neglected, hand-fed then bred, and abused birds including cockatiels, mini-macaws, conures, a mealy amazon, CAGs, a B&G macaw, and cockatoos. The only bird it didn't work on was Winston the Severe Macaw, and I later found out that was because he hated women. Today he is his (male) owner's best buddy.