First Steps
Obviously, the very first thing to happen is to get hired/picked/whatever. I'm leaving this part out on purpose. If you've had a job, you probably know generally how this part works. If you don't have a job, watch this for some handy tips.
I like to have an idea of what the director's looking for before I read the script. Nothing specific, just basic questions about the balance of realism and presentationalism. (wikipedia acting article helpful if you can't tell the difference) Some other basic direction is often helpful.
Contrary to popular belief, designers do, in fact, read the script (usually). (kidding. mostly.)
Design teachers say you should read the script a bunch of times for lots of different things. Each reading should have a singular focus. For example. the first reading should be for entertainment only.
This doesn't really work for me. My brain goes too many places at once. I inevitably find myself making note of things from a design standpoint when I'm supposed to be pretending to be an audience member.
In any case, I read the show and let it percolate in my brain a while. I'll probably do a second reading from another headspace where I'll allow myself to jot down a few notes.
By this point I generally know where I'm going with the set design. So it's time to sketch!
I tend to work in notes and quotes from the show with my rough sketches. Directors don't always see these. I may want to hide parts of them or they may emphasize incorrectly directions I don't actually intend to take. In this case, I did show my first roughs to the director.
These I did with one reading, and a skim for reminders and themes:
So not bad, but not totally solidified.
So I read it again, and sketched again.
Now I wait for feedback.