There are three basic starting points when you are asked to put together a promotional piece. You could start with the music, the shots, or the script. Each starting point is valid depending on what you are putting together.
1. Music: If you have a great piece of music already, or you don't have one yet and need to make it still, this can be a great place to start with a promotional piece. Music is a very powerful tool for helping people feel a particular way, and can also help you decided how to cut your piece.
2. Shots: If you know you're going to be putting together a quick montage from a larger project, it can be helpful to start by narrowing down your footage. This will allow you to simply throw away anything that isn't super interesting to you, and can help you decide what kind of music may best serve the shots you've kept.
3. Script: If you know there are very specific things that need to be said or conveyed in your piece, it is a good idea to start with the script. Even if the script doesn't dictate each shot for you, it will allow you to look through the footage and music selections with a clearer picture of what you want to use.
Each of these starting points is just that: A starting point. Each of the three is perfectly viable depending on the type of promotional piece you're producing. I've used all three at one point or another. It all depends on the project and what I know I have to work with.
And that's important to keep in mind as well as you work on projects: Start with what you have, and work from there. If you have a script, start there and assemble the pieces you need. If you already have a selection of music, start there. And if you've just got a pile of footage, start by cutting that down to the few gems you love.
Your Media Production Mentor