You teach a youngster to box by teaching a set of principles and a set of fundamentals that support those principles. You give them the tools and you let them develop an instinct for fighting. They need to find their own way, hit and don't get hit that is the game, give them the tools let them find their own process. Ultimately boxing is an art and each fighter must find a way to express himself in the course of a fight. No two fighters will do it in exactly same way.
You do not teach a cookie cutter offense and defense and have them "do " them at each other. “Do a 1-2-3” no! A teacher shows them how to develop a flow, “don't think feel!” The young fighter has to get out of their own way in order to enter into "boxing mode" flowing seamlessly from defense to offense and back.
A fight is like a symphony it has beginning, a middle section, and an ending. It is being written and played at the same time. A fighter must develop a sense of fight progression, have contingencies planned for each stage, and be able to change pace or tactics as the story unfolds. This requires mental focus, concentration, during the entire contest.
Inexperienced fighters don't have this sense of the big picture, they can't create on the fly, it is all happening too fast. It takes a lot of rounds to be able to write and conduct the symphony while it is happening and then change with the flow of the fight. This is called "ring generalship" and it really can't be taught, it must be an instinct in a fighter. I recommend that you study early Sugar Ray Robinson fights to learn about dictating pace, rhythm, and control of the center of the ring against a skilled opponent.
An important part of ring generalship is an awareness of what the other fighter is experiencing, is he tired? Does the pace suit him? Is he hurt? Is he frustrated? or is he in his comfort zone? what is his perception of how things are progressing? All of this must be taken into account in real time while the fight is ongoing. While this is where a good corner helps, the personal experience of the fighter is the key.
Trainers at Stillman's Gym in NYC in the 30's and 40's developed the concept of the "defensive fighter" stressing balance, foot work, head movement, and angles as well as punching. The “dean” of those old trainers Ray Arcel said that the key to this idea of the defensive fighter was the coordination of head and foot movement that he called "slide and roll." He taught fighters like Barney Ross to bend from the waist, roll under a punch, and slide over to a punching position where you can not be readily hit while you get off with both hands. Reduce the amount you get hit while increasing your offensive output, this was the defensive fighter.
Charlie Goldman had an entire chapter in the book, Boy’s Book of Boxing and Body Building, co-written with Rocky Marciano, on the defensive fighter. He even used some of the same terms that Arcel used.
In the book Goldman said,” A skilled defensive fighter is usually the product of long and intensive training. Too many of the offensive boxers you see today are green, untried youngsters who do little more than throw a barrage of reckless punches. The skilled, careful boy knows that slipping punches is also part of the skill of self-defense.”
In Chapter 6, "How to Begin", after the section on how to make your own heavy bag, Rocky Marciano says;
“When everything that can be said about boxing has been said, one fact will stand out above all the rest: the best boxers hit more and get hit the least. This refers not only to the number of punches, but also to the strength behind them.”
It is this simple fact that a beginner needs to grasp first before going on to learn the art of self defense.