Your party Pokemon must have 10 different ribbons (i.e., 1 could have 10 different ribbons, or all of your Pokemons' could equal 10. Either way, they must be different.).
If you haven't yet entered the Hall of Fame, you must do that first, then go to the outskirts of Snowpoint City and ride the boat. Make sure you have the National Pokedex(see 150 Pokemon, enter Hall...
Get a Pokemon with 10 ribbons. To get ribbons go to the contest hall in Hearthome and for defeating the elite four, also if you west of Sunyshore City along the beach you will see a house (Dr....