How Do I Create Scenarios?

Hi folks.
I'd like to ask how I could create some scenarios on some routes I own.

Eg, I want to create a scenario going from Derby to Nottingham using a specific train and stopping at specific stations but also have regular ai passing or waiting at platforms.
How do I go about doing this?

Please bear in mind that I have used the TSW2 scenario planner but as a newbie to TS21 I'm only starting to learn about various features.

 

It takes a bit of explaining but if you watch this then ask the questions that might come up

 

This one also makes it very plain... but it IS a bit long winded.