Start planning top down - "helicopter view", although there are no hard rules here - use any planning tool/methodology that works best. Pen and paper will work to - create a storyboard and do a walk-through: if there are gaps or broken links in the flow - they will become more obvious.
Most critically important things to decide at the very start are the goals and the content.
What are the goals of the training session(s)/programme? How do I define goals?
Ask yourself a question: “What do I want the participants to do after they complete the training?” Think about it in specific actions (rather than generic terms), i.e. “participants should be able to review/replace/install/contact meus, etc”.
And your answer will be the definitions of goals that training is meant to achieve.
Pitfall to avoid:
Focussing on your own actions (“I will do the presentation, then I will do the demo, then…) and losing the site of goals.
Plan your own actions but keep audience in mind at all times, i.e. “how my actions will help to achieve the goal”goal?”
Taking stock of the content: do I have everything I need for the training?
Sample content inventory: