Instructions
Start "Pokemon Platinum" and walk in grassy areas to meet wild Pokemon. Even if you don't battle or catch them, the Pokedex collects their data because you saw them. If you catch a Pokemon, more data about that Pokemon appears in the Pokedex, and you gain a team member to use in battles.
Battle every trainer you see in the game. Often they are evolved forms of Pokemon you aren't likely to meet in the wild. Seeing the Pokemon in battle updates your Pokedex.
Play the "Pokemon Platinum" game through to the end. By now you will have only a few unseen Pokemon left in your Sinnoh Pokedex. Battle and defeat the Elite Four and Champion Cynthia to receive the last bits of data you need to complete the Pokedex.
Fly to Sandgem Town and visit the Pokemon Lab.
Show Professor Rowan and Professor Oak the completed Sinnoh Pokedex. Professor Oak will upgrade it to the National Pokedex for helping them with their research. The National Pokedex allows you to gather information about Pokemon species from other regions.