Before you even start, they should know that you only can reap what you sow, or in modern english - you only get interest AFTER you invest. They need to learn that YOUR time is worth soemthing and that when they don't practice, they are wasting your gift of time. I consider attitude to be more important than the mechanics! Once you have their attention, a daily routine with breathing, long tones, slurs, scales, tunes and technical studies are critical to building and maintaining. I don't know how well you play, but if you are proficient, you know what works for you. Start with that. If you are not very good, get lessons before you mess somebody else up. I have had quite a few students that prove what is possible when the deaf have been lead by the blind: bad habits that some never recover from.