Go to CRScrafts and buy a restringer - it's basically a hook on a handle that you use to reach down into the doll to pull the elastic cord through the neckhole. Or you get some good stiff wire from the craft store - like thick plant wire - and bend one piece in a U shape using that to pull the elastic through the neck hole.
take a peek at how your new doll is strung - they are all basically the same (although I highly recommend putting S hooks on your BBB - the hands and ankles pose better that way.
Also search "String BJD" on you tube and I'm sure you'll find several videos.