Instructions
Walk into a large patch of grass. While the Poke Radar will work in small patches of grass, your chances of finding a rare Pokemon increase if you look in larger patches. Examples of ideal areas include the western and eastern side of Route 204, just west of Route 227 and south of Pastoria City.
Press "Start," scroll down, and click "Bag." All items in your bag will appear.
Highlight "Poke Radar" and press "A." Select "Use." An area in the grass will shake. Run to the area and you will immediately encounter a Pokemon. Continue to use the Poke Radar until you catch the desired Pokemon.