THIS only works on Pokemon Platinum... Go to Floroma city. Talk to the lady next to a sign. She will give you the flower.
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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.
If you used an AR to walk to the flower paradise then you will not see Shaymin. Nintendo decided to get on top of that ability to just walk to shaymin. You have to get Oak's letter on mystery gift and take it to the guy in Victory Road. He will let you pass and you can go get Shaymin.
You can find him if you trigger the Shaymin event, where he asks you to write down the name of a trained that inspired you to open Seabreak Path, which leads to Flower Paradise, where you catch Shaymin.
You need to have Shaymin in your party while doing this. It only works if the Shaymin was given in an event or you used Oak's Letter to catch Shaymin in it's natural habitat in Flower Paradise so if you just hack it off using a cheating device, it won't work. As soon as you have Shaymin, go to Floaroma Town and talk to the girl near the sign. She will give you a Gradicea Flower. The flower is used to turn Shaymin into Sky Forme without trading it into Pokemon Platnum when you use it on Shaymin. And since it's a key item, you can turn it back it into Land Forme if you want to.
Once you have Shaymin in your party, go to Floaroma town, talk to the blonde lady outside the flower shop and she gives you the Gracidea Flower. Then, give the item (which will be in the Items section of your bag) to your Shaymin and by night, it will have changed into Shaymin Sky Forme. Hope I helped! Oh yeah, you need to have Oak's Letter to see it, and then you have to chase and catch it through Seabreak Path, which leads (yes, the path is veeeery looong so it will be faster to use a bike) to a place called Flower Paradise. Hope I helped!