Instructions
Visit Tom Nook's Shop throughout the month. Because the towns in "Animal Crossing" are procedurally generated, his shop is in a different location for every player. You get raffle tickets for each purchase you make.
Go to Tom Nook's Shop on the last day of the month. Instead of selling items, he conducts a raffle. You'll see three prizes on display behind him.
Speak with Tom, and he'll offer to enter you into the raffle. Agree to enter. He'll take five of your tickets and then draw a ball. If it matches the color of one of the three prizes, you win the prize.
Continue speaking with Tom until you've used all your raffle tickets. Unused raffle tickets become invalid when the month ends, so use all of your tickets in the same month you acquire them.