From being a student, and also a teacher of mindfulness, I have learned a few things that I would like to pass along to 'wannabe' teachers:
1. Have integrity as a teacher
Learn from a reputable teacher. Sit every day for 30 minutes at least, and go on a long silent retreat at least once a year. Do not follow someone else's script. Instead let the words flow from your in-the-moment experience and your own practice. If not able, better have your 'student' listen to a recording of a more experienced teacher.
2. Do not add to the moment
Mindfulness practice is simply about being aware of what is. It is not about visualizing what is not there, or forcing your breath into a different rhythm. Those techniques belong to other types of meditation practices with a different goal.
3. Stay away from 'I' and 'You'
Instead go for 'we' statements, or even better, action oriented instructions without personal pronouns, e.g. "body sitting still, being breathed", or, "turning our attention to the experience of hearing sounds", etc. This way, the possibility of experiencing not self gets introduced.
4. Leave the space open for the wide range of possible experiences
Do not impose your idea of what the now ought to be. During body scan, make room for possibility not just of sensations but also of no sensations. During mindfulness of emotions, give examples of many emotions, and also possibility of not knowing. Also, do not tell people that they should not think - such a common misconception, that get unfortunately passed on by so many untrained 'teachers'!
5. Talk, but not too much
Guiding means you need to use verbal guidance throughout the meditation to hold students' experience. It does not mean placating the whole time with non stop talking. You want to give students a chance to take in your instructions, and then experience for themselves.
With deep gratitude to those teachers from which I learned much about the art of teaching mindfulness: Gil Fronsdal, Bob Stahl, Jon Kabat-Zinn