For a child to gain self-discipline they must first understand and experience discipline.
Discipline does not necessarily mean punishment.
Punishment without wisdom gained will provide no discipline.

To learn self-discipline a child must be subjected to others in their life using self-discipline.
You teach by example.
While teaching, you explain the wisdom behind your behavior and why it is important.

If you ignore your child, your child will learn to ignore you.
If you scream at your child, your child will learn to scream at others.
and so on and so forth.

If you want your children to be on time and hold their promises, you must be on time and hold your promises.

Your children are mainly the product of your own teaching.
Granted, there are many other influences but being YOUR child means its YOUR responsibility to teach the important stuff to your child.
Don't expect them to figure it out on their own.
Teach them.

You can teach them on purpose or you can teach them by example.
So, if you have low self-discipline, its not a stretch to imagine your kids do too.

Then dealing with teenagers it helps to practice nailing jello to a tree in a windstorm.

Thx that makes sense