‘What would you do?’ – it’s a small point but makes a big psychological difference as it places you outside the story. We often say we would do something in a situation knowing full well that we probably wouldn’t. With a scenario we’re looking for the engagement of the learner’s thoughts and emotions so that a response will be more genuine. So we don’t want the distance of ‘this isn’t me but if it was, here’s what I think I’d do’
Your manager may be worried that it's over-complicating things to make a scenario. They may be right. If the task you're training for doesn't involve judgements and decisions - or is just knowledge not a task - forget it. But if people have to make judgements based on a principle at work, they need practice to get it right and learn from mistakes. That's what scenarios are for - they're worth the effort because they work! More reasons here.
Here's a free tool you can use to build your scenario and instantly create a working prototype: Twine.
There are two ways - plan it top down, from flowchart to actual words. That's the best way. But it's also possible to write it 'bottom-up' in other words create the story and let it unfold as you go. I've done both and both can work. More about the methods here.