Instructions
Complete your Johto Pokedex by seeing all 150 Pokemon in the region, then go see Professor Rowan at his lab in Sandgem Town, near where you started the game. He will give you the National Pokedex, and will install the Poke Radar application on your Poketch.
Select the Poke Radar in the "Key Items" section of your bag. Assign it to the "Y" button.
Run into a patch of long grass where you are normally ambushed by Pokemon --- don't ride your bike, or the Poke Radar won't work.
Press the "Y" button, and some patches of grass will start to shake. The harder a patch of grass is trembling, the more likely it is that a rare Pokemon is hiding inside.
Run into a shaking patch of grass, and a wild Pokemon will jump out and battle you.
Finish the battle by catching or knocking out the Pokemon, and you will see some more patches of grass start to shake. The furthest patch from you will contain the same kind of Pokemon you just battled --- good information to know if you want to build up a chain.
Leave the grassy area to stop using the Poke Radar.