Step 1:

Heat the balsamic vinegar in a saucepan over medium heat. Bring to a bubble. Let the balsamic reduce by half. It will become thick and syrupy. Let cool.

Step 2:

Chop the watermelon. I always buy it sliced because it saves time, and because I don't have a car and really really don't want to carry an entire watermelon home. Combine the watermelon and feta in a bowl. Drizzle with the balsamic immediately before serving. The watermelon will soak up the balsamic, which tastes great, but doesn't look too appetizing. This dish is much prettier if the balsamic is added at the last minute.