The best way to learn how to write training is to experience it for yourself or, at the very least, observe.The most honest way to do this is to select the martial art that you want your character training in and find a local school that is willing to let you sit in and watch their training sessions. It’s common practice in many schools to open up their classes to prospective students. Ask the instructors in charge specific questions about training (even if you think they’re stupid) and about training teenagers. I suggest this because while there are base similarities in how to prepare and teach both body and mind, each style (and each school) often have unique perspectives on what works best for them. The only way you can know what those are is either by asking or by experiencing it for yourself.
In most non-training story narratives, writers have a nasty habit of going too hard and being too brutal. Many seem to believe that all training works the same way as R Lee Ermey in Full Metal Jacket. R Lee Ermey was a staff sergeant in the Marines and his style of teaching was specific to both the Marines and the 70s. Unless you’re specifically writing a Vietnam War era drill sergeant, I suggest looking elsewhere.
Training is not a mystical mysterious experience, it is at it’s core all about a teacher and a student. It’s about learning much in the same way you do in a high school or college classroom except that it involves physical activity. The best way to write a training sequence is to discover what is being taught, what knowledge is being imparted to the students, and how the teacher is choosing to teach the student that information.
A good instructor will push a student past their self-conceived physical limits and out of their comfort zone without pushing them past their actual physical limits. Unlike in Divergence, no one will be left to flounder and guess at what they should be doing. Techniques will be shown to the students by the instructor and then the students will be asked to perform them under the instructors supervision. They will repeat the technique through a series of repetitions, often breaking it down into pieces and performing it on a count so that the student develops a full understanding of all the pieces of what they are doing. On each count they will be asked to hold position while the instructor and their assistants check the students’ body positions and make corrections. It will be slow and, for many students, it will be frustrating. Expectations will often be dashed when faced with the slow accumulation of knowledge, but that is also important because it teaches the student patience and respect for what they are learning. Humility, patience, perseverance, and generosity of spirit are all qualities that the student is being taught as they learn to fight. Learning when it is appropriate to fight is as important as learning how to fight. This is true of both Eastern and Western martial arts, where the student is taught to fight in defense of themselves, defense of their home, and defense of their homeland.
The difficulty with writing a training sequence is that the author has to be a teacher. It’s their job to communicate how something is done to the reader, not just to the characters. In order to write an effective training sequence, you yourself have to be an authority on the subject. This is part of why I feel the Karate Kid remake actually works better at this aspect than the original because Jackie Chan was teaching Jaden Smith during production and they developed an authentic rapport. This is also why Tamora Pierce’s Protector of the Small quartet and Trickster’s Choice/Trickster’s Queen novels are successful because they have done the research but ,more importantly, knew what the end product of the training would be.
We don’t train someone for training’s sake, we are training them to do something. Once you figure out what the end goal of the training is, then you can limit your search to the appropriate skill sets and venues that specialize in what you are trying to create. Once you know that, you then do extensive research on the subject until you understand everything you can about it. Then, you can write your scene.
So, ask yourself some simple questions:
What am I training my characters to do?
Does what I’ve chosen for them to learn match up with what I want them to do (for example, if you want your characters to be aggressive fighters then aikido is not the right choice)?
If yes, then awesome. If no, then does what they are being trained to do make sense for what their culture expects or requires of them? Do they feel it’s something they need to be learning?
What are the skill sets the real world professions require? (If you’re having trouble figuring out the above this might help to get you started.)
I hope this helped answer your question. I know it’s a little long winded and roundabout.