Instructions
Collect all 150 Sinnoh Pokemon in the game. For greatest success, consult a checklist or a guide, like the Sinnoh Dex Guide, which is listed in the Resources section below.
Return to Pal Park, from the start of the game, and speak to Professor Oak. He will congratulate you on capturing Sinnoh's native Pokemon and award you the Pokeradar Checker as an application for your Poketch.
Press "Start" and access the "Key Items" menu. Assign the Pokeradar Checker to "Y," then stand in an open field and press "Y." This will activate the Pokeradar Checker and increase the chances of a rare, non-Sinnoh native Pokemon appearance.