The performance takes place on a portable stage that's pulled to a location in DCA - we saw it in front of the lake, before you get to the rocket swings. The performers walk alongside the stage until it stops then hop on stage and start their performance. They use both the stage and most of the sidewalk, have props, sing, and pull several kids out of the crowd to help with the show - mostly sports. The version we saw a couple of weeks ago was from HSM2 - they did "Batter, Batter, Swing and in December we saw them do "Get Your Head in the Game" - that's the one where my grandson was picked. It lasts less than 15 minutes. I think it's offered one other place in DCA, closer to the entry, maybe by the big sun but we've seen it both times toward the back of the park. I know it's done in the afternoon and I'm pretty sure it's also done in the morning. Lots of energy, lots of music - loud music, lots of fun. Hope that helps.
Just a note about the signs. We had a sign when we were there the beginning of March. Nothing fancy. Something about the force is with me. My DS 11 was holding it in his hands when we walked through security. We hadn't thought to roll it up or stick it under stuff in the stroller. At security they wouldn't let him bring it in because Disney has a policy about NO signs of any kind. Even though they could read it and see what it said. DS was sad - got picked for Jedi Training anyway. Littler DS 5 did not get picked - very sad
We did not have star wars t shirts, just enthusiasm. But t shirts seemed to really help you get picked.
And sometimes being a little cute girl really helps too