My own experience of writing for the theatre is to think this way. First of all, come up with a clear vision of what the finished production will look like. This is what I am doing for the first show that will be produced through Grid#Productions.
I used the same process with my first piece, Cinderella. Okay, so the story is very familiar. However, I wanted it to be a simple show that could be produced by an amateur theatre group on a very tight budget. In fact most shows can be produced very simply.
Once the vision is there, the rest sort of falls in to place. It is a matter of introducing the various characters and ensuring a good plot line that has a beginning, middle and an end. This is all rather similar when starting a business - putting on my hat as a business adviser for a moment. It's all about that vision of where you want to be. At the end of the day, if you can see the big picture, the detail will fall into place much more easily.