The first step for me is to get motivated.
If I can find a reason to get excited to start something the next steps become so much easier. This is usually a moment in a show that catches my imagination while I read it, or even trying to solve the puzzles that the playwright gives us. Such as in Urinetown, I found that my motivation for the show was finding out what made each side tick.

The second step is to gain my own direction.
Once I have solved the issue of getting motivated I need to find a problem to solve. I can usually do this by defining parameters by which to contain the design, this creates an isolated problem in which I can work from many directions to find a solution. To define these parameters I like to break down ideas in the show   into one concise, poetic statement that defines some aspect of the show (action, mood, themes). I can use this as a grounding for my view of the show while talking to designers and directors.

These two steps lead to a great place jumping off point to create any kind of design. It boils down the show to the essence of what is happening to constrain it to the text, while be flexible enough for the director to make decisions on how the overall productions will look.

I have found an issue with this beginning process, I will create an analogy within the concise summary statement that is difficult to alter if the director views the show fundamentally different than I, which of course plays into my natural stubbornness.