- Find someone elses Entralink
- Find them in Black City
- Invite them to White Forest
- Presto, you have grass in White Forest!
It's not the best guide ever, but I haven't done this before. I can just use the PokeTransfer lab, it takes less time and is easier.
Like Pkmn said, look in the booklet. It'll give you a more detailed answer to your question. But two precautions - the other person must have caught Reshiram and completed what Game Freak considers the main game, and they must have Black version. Make sure these two requirements are met and invite everyone you want.