We've done more than a few, they can be a lot of fun! But, it's total luck of the draw. You'll usually play a first round with around 20 other people all at their own slot machine. For 2-4 minutes, you'll press the spin button as fast as you can and build up a score. Top 3 (typically) go on to the next or final round where you'll do it all over again. Usually, they'll pay the top person in slot credits as said above, but they may pay the top 3 people.
There are usually multiple tournaments throughout the cruise. On our last trip, the pre-purchase is essentially a coupon to turn in for the tournament you want to be in. You'll turn it in at the casino cage for a "token" to give to the tournament host. Sign up usually starts 30-60 minutes prior to the start time.