I undertook a substantive Supervision training to Diploma level via Cambridge Supervision Training. I have experience of supervising trainee Counsellor/Psychotherapists (for which a previous role as co-tutor on a local counselling course has also proved helpful), and trained therapists.
I also supervise people in other helping professions e.g. doctors, nurses, care-workers who find supervision provides a useful space to reflect on their practice and any impact on themselves.