Critical in the first instance is competence in HSE, so here you need to have suitable qualifications e.g. MSc (in HSE related subject), NEBOSH, NVQ in Occupational Safety & Health etc. You must have the experience of applying HSE in the workplace and be familiar with the critical components of legislation, management systems, with an awareness on behaviours and cultures. There are more aspects, but this is just a first point. Secondly comes the ability to train, which is not as easy as many think. You need to understand the methods of delivery, understand learning styles, awareness of learner needs and have the ability to adapt to them. Understand the phases of training, introduction (aims and objectives), the learning phase (what they need to know) and the evaluation stage (have they met the learning objectives) - again a simple methodology. Qualification in PTLLS (Preparing to Teach in the Life Long Learning Sector) or Professional Training Certificate help you with this. I can go deeper, but at this stage, would leave it at that.