The prank will start off with someone from the crew like Ehren; someone who will not be recognized.
Go into Walmart and grab a shopping cart and move through the store filling the cart with random items. Completely fill it.
Then go to the express 10 items or less checkout at first and if they don't let you checkout there, just go the normal checkout. Then when it comes time to check out place all the items on the counter then say "Oh I'm sorry I forgot an item -- be right back."
While this is happening have someone completely fill another kart of items and bring that back to the counter. Repeat this 3 times. After the cashier completely bags everything check your pockets then tell them you have no money and you will not be able to purchase the items.