First choose your OWN case. That will be the case that you will save. Now it will tell you to open some cases. Try to guess how much money YOUR case has by opening the other cases Then, when you open enough cases, there will be a bank offer. You could choose DEAL, and accept the offer and end the game. You could also NO DEAL, which is when you think your OWN case has more money than the offer. Once there are only two cases left (including your OWN case), and you choose NO DEAL, you have to choose between your OWN case and the other case. When you click on one of the two cases, you will get the money that the case holds.
If you didn't understand, you can look up the rules. Found this game in the arcade, pretty easy to build.