Depends on the game, dude.
Platinum:
Go to Floaroma Town (North of Jubilife). Then, talk to a girl outside the Berry/Flower Shop. She will give you the item caled Glacedia's Flower. Then, catch Shaymin in the Flower Paradise. Now, give Shaymin the flower. It will become Sky Form Shaymin now. It also becomes a Grass/Flying Type. To revert it back to normal, remove Glacedia's Flower from Shaymin.
HeartGold/SoulSilver:
Take your Shaymin to the flower shop in Goldenrod, and you'll get a Gracedia flowe
A Shaymin in Shaymin Village gives you a Gracidea. Use that while in a dungeon and he will change form.
yes
That Is A Stupid Glitch If This Happens You CAN NOT Get Shaymin "sky form" :( unless your using it during the night time then its just that shaymin cant turn into its sky form at night
By tweaking you can access the Flower Paradise. You can catch Shaymin here. After catching it, Fly to Floaroma Town. A woman there will give you Gardenia Flowers giving these to Shaymin will turn it into a Sky Form.
You need to get the flower, by taking a LEGIT shaymin to the flower shop, talking to a certain girl, who will give you a flower of which you must use on the shaymin. It will work if the shaymin is LEGIT.
A "Gracidia Flower" is what makes a Shaymin turn into its sky forme.
Shaymin's sky form is a form that the Pokemon character Shaymin can take.
in soul silver and hart gold go to goldernrod city go to the flower shop and have shaymin as your lead Pokemon and walk trough so shaymin apears out of the pokeball and the first person tells you it so cute it shaymin and then he gives you the gracidea flower then you can change it into sky form. from Little B
Shaymin reverts to its normal form at night. Wait until the day to make it sky forme again
get a Gracidea in the shaymin village in a dungeon use the Gracidea on shaymin + shaymin is in sky form effect removes if your at the next floor and sky shaymin sky forme is on double speed in Pokemon diamond,pearl&platinuam you get an gracidea,go to the Pokemon league then,there!
Sky's Summit (Summit)
to get sky shaymin, you first must get an event shaymin (have to have oaks letter shaymin) then you have to have it first in your party. Go to floaroma town talk to the lady in purple on the dirt road she will babble on about the gracidea flower and shaymin then she will give you a gracedia it is in the key items section, use it on shaymin during the daytime when it isn't frozen, it will have a short scene and it will be sky forme to turn it back put it in a box at the PC, freeze it or wait until night time