i think you need an action replay but i don't have one soo...
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Get Oak's letter and run across Seabreak path until you are in Flower Paradise and talk to Shaymin battle and Catch it if you like.
When you come to route 224 (Proffeser Oak wants you to in Oak's letter) you can thank anyone you want. It doesn't matter. After that, shaymin appears, and you chase her down seabreak path. Make sure your Pokemon isn't a high level, though. Shaymin is level 30. Hope this helped! ( P.S: use your bike on seabreak path, it's LONG!)
near the Pokemon league
near Ravaged Path
You have to go to it but nothing happends