There's a woman in the pokemon center in Lacunosa Town who talks about gracidea flowers. Bring her your Shaymin (NOTE: It must be an event Shaymin with a "Fateful encounter" tag! Hacked shaymins will not work!) Then, she will give you the flower which will let it become sky form. Note: it can not become sky form if it is nightime or it is frozen.
The question already was asked here as well: