Getting the PokeRadar and Trainer Counter
At the beginning of "Pokemon Platinum," Professor Rowan instructs you to go on a Pokemon-catching adventure. Hundreds of Pokemon live in the Sinnoh region, but you only need to see 150 of them to please Professor Rowan. Once you see at least 150 Pokemon, visit Professor Rowan's lab. When you talk to him, Professor Oak enters the lab and gives you the National Dex. After this, Professor Rowan congratulates you with the PokeRadar. If you want to chain enough Pokemon to catch a shiny Pokemon, you also need the Trainer Counter. To get it, go to the Pal Park and talk to Professor Oak.
Chaining Basics
To chain Pokemon, head to a grassy patch anywhere in the region. Register the PokeRadar as a key item, then press "Y" to activate it. When you see a shaking grass patch, walk into it to fight the Pokemon hiding within. Repeat this process by fighting Pokemon of the same type. This forms a chain. If you run into one Pokemon of a different type, the chain breaks. Using an item called super repel keeps random Pokemon away from you during a chain.
Catching a Shiny Pokemon
When you chain Pokemon, the ultimate goal is to capture a rare Shiny Pokemon. Increase your chain by fighting Pokemon who hide in moving grass. Use super repel to avoid breaking the chain. If one dose wears off, use another immediately. Once you chain 40 Pokemon, press "Y" to reset the PokeRadar. Look around for grass that flashes twice and does not move very much. This grass hides a shiny Pokemon. Walk into the glowing grass patch to catch your hard-earned prize.
Advanced Chaining Tips
Some grass patches only shake, while others shake violently and sparkle. If you start the chain with a sparkling grass patch, never touch a grass patch that doesn't sparkle, as this breaks the chain. Look for shaking grass patches in the middle of the field, and avoid patches on the edges of the area. Touching patches on the edge can break the chain. Find Pokemon swarms for easier chaining. Swarms are large groups of the same type of Pokemon. Your rival's sister in Sandgem Town tells you where to find a different Pokemon swarm each day.