I don't always work in that direction. Usually, I just write the full review, which is why I haven't posted as many First Impressions as I told myself I would when I first floated the idea.
But sometimes, especially if I'm having trouble getting a review started, I'll write the blurb first. (Blurb = the 100- or 200-word capsule review that will appear in CP's listings during a play's whole run.)
Or I'll bullet out my ideas about what I liked and what I didn't, and try to tell myself in 25 words or so what the play's about and what I thought of it. That helps kick-start the process. Certainly it did this week; there's a lot to talk about with a play like Arturo Ui, and sometimes I just don't quite know where to start.
Of course if all else fails, I fall back on the Mondello Maneuver: Pick a moment, any moment, that was striking, and describe the sh*t out of it for 100 words or so. Even if you didn't like the rest of the production, you love that moment, at least long enough to do it justice.
Learned that from Himself back in 1996 or so, and it never fails.