Instructions
Complete the Sinnoh Pokédex by encountering all 150 Pokémon in the region. You don't have to catch them all; simply seeing them will suffice.
Visit Professor Rowan in Sandgem town; he'll give you the PokéRadar item.
Equip the PokéRadar as your default key item.
Walk into some grass, without using the bike, and either press the "Y" button or select the PokéRadar from the Key Items menu.
Head towards any patch of grass that is rustling; the rustling indicates the presence of a Pokémon. The harder the grass is rustling, the more likely it is there is a rare Pokémon inside of it.
Defeat the Pokémon. After the battle, you will see more patches of grass rustling. Head for the furthest away patch if you want to battle the same kind of Pokémon again and build up a chain of victories.