First you must have:



For Ho-oh you need:

Nintendo Gamecube
Pokemon Colloseum
Memory Card
Pokemon Ruby, Sapphire, Firered, Leafgreen or Emerald game pak
Gameboy Advance 
Gamecube/Gameboy Link Cable
Nintendo DS
Pokemon D/P

Requirements:

You must have beaten story mode on Colloseum
You must have beaten The Elite Four on Pokemon Game Paks
You must have the National Dex on Pokemon Diamond or Pearl

Instructions:

First, on Colloseum go to Mt. Battle with your greatest team. (bring your master ball if you still have it or ultra
balls)
Then, beat all 100 trainers.
After that, Ho-oh will appear for you to catch.
After catching Ho-oh, trade it into your game pak.
Then, put your game pak in the second slot of your DS.
Migrate Ho-oh (make sure ho-oh has no HM moves) and five other pokemon.
Ta-da you have Ho-oh!

Tips:

If you have Emerald, clone the Ho-oh in the Battle Tower. (Helping links at the bottom.)
So you can have a Ho-oh army!!