Instructions
Catch a Roselia. You can find one on Routes 212, 221, 224, 225 and 229, as well as in Trophy Garden and Great Marsh.
Find a Shiny Stone. You can find one in the last room of the cave on Iron Island and on Route 228. Pokémon with the Pickup ability that are Level 41 or higher have a chance of finding them randomly.
Select the Shiny Stone and use it on Roselia. Roselia will evolve into Roserade.