Target training is a fabulous tool, and it's easy to teach. The concept is very simple: the bird touches a "target", usually with it's beak, and it gets a reward. Target training can lead to improved owner-bird communication, can be used to shape other behaviours, and is extremely helpful for aggression, phobias, and may other issues. Even birds who cannot be handled can be successfully target trained. One trained, most birds would do anything- including climbing or flying over anything in their path- to get to the target. This way, you are in control of where your bird goes without directly handling them. There are dozens of ways this can come in handy during the lifetime of your bird.
The requirements for target training are very simple. You need somewhere for the bird to perch; in or out of it's cage, it doesn't really matter as long as both you and your bird are comfortable. You will also need a target; a thin dowel perch held in the hand works well. You can put red food dye on the tip to designate the "target zone", but this isn't strictly necessary. You will also need some way to reward the right behaviour; what reward depends on your bird. A good reward is anything your bird will be eager to work for, and may include small pieces of food or treats, scratches, toys, etc. Food or treat rewards should be tiny, so your bird doesn't get full too quickly, and eaten quickly, so your bird doesn't forget why they got the treat in the first place. Experiment with different rewards, and find out which your bird prefers. Some birds become bored if the same reward is offered again and again, so if you can find a couple things your bird will work for, all the better.
Target training is taught through approximation and variable rewards. This means that your bird gets a small reward for every step or action they take that gets them closer to the behaviour you want, and big steps forward get bigger rewards. the big rewards are referred to as "jackpots". In the first stages of training, simply looking at the target may get them a reward if they are a shyer bird. In more gregarious birds, they may only need to be reward when they actually move towards it. If at any time your bird seems uninterested or doesn't volunteer the next step, step back, wait a moment, then go back one step and try again. If they still don't offer the behaviour you want, you may want to consider asking for a smaller approximation or with a more desirable reward. Do not move forward until your bird is consistently and reliably completing the previous step. Remember to remove the target after every try, whether your bird completes the step or not. You want to train your bird to touch the target on cue (the presentation of the target), not to touch the target whenever they want a reward!
If you are working with a bird that may bite, remember to keep yourself safe. In these cases, petting is probably not going to be a good reinforcer. Food and toys are best offered on the very tip of your fingers, far enough away that the bird has to stretch to get them. If you bird is already stretched to get the treat or toy, it is unlikely they will lunge to bite.
Many birds take to the target right away, but some require encouragement. Keep sessions short and gauge them by your bird. Is your bird still interested and engaged? If not, it may be time to stop. Short training sessions every day are preferable and yield quicker results than longer sessions only a few times a week. With a bit of patience, the right rewards and some time, your bird will quickly be scrambling to get their beak on that target. Have fun, and good training.