One year I got asked to be in it and I was so excited. My job was 'lu.' It was a pretty tricky little dude. We would chant "ha-lellujah DOWN" so our cards would all flip down simultaneously. These rehearsals were so much fun. When you're standing on a chair, in a robe, flipping cards to the Hallelujah Chorus in the 10th grade with your best friends, it's hard not to get tickled.
It's a fun little memory and I wish I owned a video of it. But, lucky for you, there are several videos on youtube of other groups that have done it. I have to say, this one isn't as precise as ours was and I do apologize for the laughing man, but at least you'll get the drift. Enjoy!