Instructions
Getting Starmie
Enter the northernmost house in Mossdeep City. The man inside will give you the Super Rod, the best fishing rod in "Pokemon Sapphire." This fishing rod will enable you to catch Staryu.
Fly to Lilycove City and head east toward the beach. Walk up to the water and navigate to the key items pocket of your bag. Fish with the Super Rod. Press the "A" button repeatedly to reel in a Pokemon. If the Pokemon isn't Staryu, flee.
Get Staryu's hitpoints to the critical zone. The fewer hitpoints Staryu has, the easier it will be to catch. Throw an Ultra Ball from the Pokeballs pocket of your Bag. If the Ultra Ball fails, fling another. Withdraw the Staryu from the Pokemon Storage System in the Lilycove PokeCenter.
Surf to Route 124 and enter the house on the second island. Trade a Blue Shard for a Water Stone. Use the Water Stone on the Staryu to make it evolve into a Starmie. Deposit the Starmie in a nearby PokeCenter.
Transferring Starmie
Remove "Pokemon Sapphire" from your Game Boy Advance and insert it into your Nintendo DS. Put "Pokemon Platinum" in the hand-held as well. Start "Pokemon Platinum" and receive the Pokedex upgrade from Professor Oak in Sandgem Town.
Soft reset "Pokemon Platinum" and select "Migrate From Sapphire." Look in the boxes of the Pokemon Storage System for Starmie and five other Pokemon. After the migration is complete, you will return to the main menu of "Pokemon Platinum."
Select "Continue." Visit the Pal Park facility and agree to star in the Catching Show. Look for the Starmie and five other Pokemon in the Pond, Ocean, Field, Mountain and Forest. Use up your arsenal of Park Balls to capture the six Pokemon. Withdraw Starmie from the Pokemon Storage System.