Review your planner before making a coaching decision; do you have enough available time to commit toward (a) planning a coaching intervention, (b) carrying it out and (c) following through. If you don't, look for someone who is competent (as a coach and in the target skill) and who has the required time.
Assess your level of expertise in the target skill. If you can't demonstrate the competency and answer most of the questions the learner/participant might have, look for someone who can.
Decide what the learner/participant must be able to do under what circumstances and to what level of proficiency in order to meet the project's standards.
Whenever possible, give the learner/participant written pre-work to study before you begin coaching.
When you meet the learner, explain what you're going to help him/her learn (the activity and outcome) in as few words as possible (too many words tend to overload a learner's memory buffer).
Start and stay upbeat. So you won't become frustrated, easily or otherwise, expect the learner to make mistakes. Be encouraging and supportive throughout the coaching interaction.
Demonstrate the new skill as perfectly as you can and without any chatter. Just do it.
If the new ability is complicated or if it involves several components, break it down and coach them, one at a time. Work on the first component until it is mastered. Then, gradually introduce second and third components until the whole competency is firm.
If the learner/participant makes an error, simply say "My turn" and re-demonstrate it. Repeat the demo if you think it's wise, then say "Your turn" and let them immediately practice.
Don't let the learner/participant stop with too little perfect practice. Keep observing until s/he can do the new skill in context (where they'd normally perform it and with the distracters that would normally be in place).
Try to build the learner's confidence and your rapport with him/her while you're coaching. Part of the value of coaching is instructive, but much of it is motivational.