I'm a qualified assessor and used to specialise in this. Particularly in making mainstream training accessible to disadvantaged people, which meant redesigning courses so they could be delivered by alternative methods to achieve the same outcome.
My advice is to have a very clear idea of what the objective is, the desired end result, and work backwards. This should stop you drifting off on a tangent unintentionally.
Design a clutter free format and stick to it. Imagine your trainee has limited language skills.
Break the subject down into small, chronological steps.
Use visual aids, diagrams, pictures etc.
Check your training covers and meets all objectives when complete.