How do you get red scarves in Pokemon Diamond?

Go to Pastoria city and look for a guy and he will ask you if your Pokemon in the top of your roster wants a scarf.MAKE SURE YOUR POKEMONS COOLNESS IS ALMOST FULL OR HE WILL SAY THAT IT IS NOT GOOD ENOUGH. If he says that, feed your Pokemon Spicy Poffins to raise it's Coolness.

Hope this Helps!