Instructions
Earn at least 30 notes and build a Level 3 house or higher. Also, you must have a big bed and have earned eight hearts before you can get married.
Climb Mount Moon using the control stick on the GameCube controller.
Receive the Blue Feather at the top of Mount Moon. Walk to the character you want to marry.
Press the "Y" button and select the blue feather item. You will then automatically propose to the other character, and she will accept or reject your proposal.
Watch the cut scene that shows the wedding taking place, and then press the "A" button to say, "I do."