Instructions
Play the game on any mode. Set any additional options, such as difficulty, to your preference when prompted until you get to the "Character Select" screen.
Move the coin-shaped cursor around the screen using the thumbstick on the Nunchuk attachment. Press "A" on the Wiimote when the coin is over the portrait of the character you want to use -- this selects the character.
Continue to press "A" after you have selected your character to change the portrait. This also changes your character's costume during the battle.