2. What kind of efforts you should do to improve your teaching skills

I've already said that such as "years of experiences", "the talent you've had by nature", "the amount of reading", won't matter for improving your skills.
As I mentioned before, If you want your skills to improve, you must through situations below intendedly.

1. Doing a demo class in a tense situation over and over again. 2. Accepting a strict instruction from supervisors who are better than you.

You will never be able to improve your skills without two things above.

1. Doing a demo class in a tense situation over and over again.

The first is in a tense situation. If you want to be a professional dancer, you should dance in front of your master and audiences over and over again. If you are an athlete, you should attend many tournaments and competitions with tense.
Improving teaching skills also need the same things.
Such as

1. Showing your class lesson attending your co-workers. 2. Doing demo-class in your private study group 3. Doing demo-class on the stage of seminars 4. Trying to get T.O.S.S Teaching Skills and Techniques Assessment System. (*https://www.toss-kentei.jp/aboutus/) 5. Doing an experimental lesson at the other school.

That means you should do your class lessons in the situation that someone who've got excellent skills would be gazing you.
Not only just once, but also a lot of times.