I agree with what has been said above. Becoming an instructor is not just about being an excellent caster, it is in my opinion, equally if not more important to be an excellent teacher, coach, mentor, listener and observer. You must be patient and understanding and must be able to adapt at short notice. Be professional in your approach, first impressions always last. As previously stated, it is essential that you learn to cast and be comfortable with a long line and enjoy it, take your time and use your mistakes as a learning opportunity rather than a reason to doubt your ability. Learn to be equally proficient when casting off either shoulder. It will take time, it took me nearly two years but I enjoyed every minute. I would advise that when you go out on the river, practice your casting or fish. Do not try to do both, you will lose concentration, use a wool tag when practising and take your time. Try to find a mentor in your area who can guide you and provide constructive criticism. There is no point in wasting time doing the wrong thing so if in doubt ask and save yourself time and wasted energy and frustration. And as I said before, enjoy the journey, it is a very worthwhile process.
Game Angling Instructors Association