You will need:
- Ruby, Sapphire, FireRed, LeafGreen or Emerald. (Game Boy Advanced Game)
- Your SoulSilver game.
- Nintendo DS or Nintendo DS Lite.
Insert your Game Boy Advanced Game in the GBA slot at the bottom of your Nintendo. Insert your SoulSilver game in the DS chip slot.
Turn on your game, and arrive at the blue selection screen. (Where it says what you want to do, i.e. Mystery Gift, Continue Saved Game etc.) Look at all the options, until you find 'Migrate from (game in GBA slot)'. Migrate the Pokemon you want to migrate. (Remember, these Pokemon cannot be holding items)
Now, go to Pal Park in your SoulSilver game (Fuchsia City, where the Safari Zone once was) and start catching your migrated Pokemon!
Hope I helped. :)