Instructions
Play "Pokemon Diamond" or "Pokemon Pearl" until you have completed the Sinnoh Pokdex. To complete the Pokedex, you simply need to see every Pokemon in the region.
Go to Professor Oak's house in Pallet Town to get the National Dex once the Sinnoh Pokedex is complete. When you have the National Dex, save, then turn off the game.
Place the game cartridge containing Ho-Oh into the Nintendo DS' second game slot. Turn on the Nintendo DS.
Scroll down at the title screen and select "Migrate." A list of all the Pokemon you've captured in the other game is displayed.
Select Ho-oh from the list, along with five other Pokemon. (You must migrate six Pokemon; you cannot migrate only Ho-oh.)
Start the game and go to Route 221. Enter the Pal Park at the end of the road. Inside, you must capture Ho-oh. You are given six Park Balls to do so. Each Park Ball catches any Pokemon without chance of failure.
Walk to the Cliff area. At the Cliff area, keep walking around until a random encounter triggers. If the encounter is Ho-oh, toss a Park Ball to capture him.