In your Crystal game, you have to get to Ecruteak City. There is some sort of Pokemon transferring thing in the Pokemon Center. Go there. On Pokemon Blue, go to the Pokemon Center and connect like you were trading between Red/Blue/Yellow. The only thing different is that you can't trade Jhoto Pokemon or Pokemon with new Jhoto moves.
1. In Crystal, you have to have spoken to Bill in Ecruteak City. (He's in the Pokemon Center.)
2. The Pokemon you are trying to trade has to exist in both versions.
3. The Pokemon you are trying to trade can only know moves that exist in both games.
4. The link cable you are using MUST be a Game Boy Color Link Cable.