In short, you use poke-transfer.
There you can select up to 6 pokemon, play a game where you can catch them all (but if you don't, they stay on your other game), and then they are transferred into your PC.
1.) Have 2 Ds systems and go to the place where the Scientist wants to experiment (really he wants to transfer).
2.) On White, talk to him. For SoulSilver, make sure its in the slot and go to DS down-load and wait.
3.) Once it connects, you will play the game. You MUST enter 6 pokemon, but if you want to only get one, focus on getting that one and then run the time out.
4.) It will transfer to your PC. But before that, are you really sure you want to transfer it? Once you do, its on White forever (Unless you trade/start over your game).
5.) Ta-da! Its in your PC for any usage!
Hope this helped!