Firstly you need 2 Nintendo DS Consoles
They can be a 3ds/XL ,2ds, Ds Lite, DS, DSi or DSi XL
You load Pokemon Soulsilver onto DS no.1.
Then you load either Pokemon BW or BW2 onto DS no.2
You need to have beat the game on Pokemon BW/2 to get to the Poke-Transfer Lab on Route 15
It is a one way trade so its bye-bye to your Pokemon on Soulsilver
You go onto the Poke-Transfer on Pokemon BW/2.
Open DS download Play on DS no.1 where you have Soulsilver.
It should come up with a Poke-Transfer Data Tool
Now after you have downloaded it onto DS no.1 a mini game will launch on the Soulsilver.
It's like Angry Birds but Pokeball version. You just catapult Pokeballs onto the Pokemon that peek out the bushes. You have a time limit
(I find this game incredibly annoying though)
After you catch/grab all the Pokemon
You can send 6 Pokemon over at a time. If you don't catch 'em all they get sent back to SS . So no pressure :)
There will be a message popping out about how its a one-way transfer. Say Yes/Ok/Confirm
(Can't remember exactly what it does)
After your SS Pokemon should go to BW/2
This Video will help
But its not finished !
You have to go through Process of Bank.
Now assuming your BW/2 cartridge is not a digital version you'll have to download your Pokemon onto Pokemon Bank via Poke-Transporter. It is included in your annual fee for Pokemon Bank.
Its quite simple you just download the Pokemon from BW/2 onto Pokemon Bank.
The Final Point to ORAS
This video will show you how