How do i introduce a new guinea pig to the hutch?

I have a a female guinea pig who is around 12 weeks old and just got a new female 4 week old. what is the best way to introduce the new guinea pig into the other's environment? I put them in the cage together and the older one constantly tries to chase and bite the new one. i was told by the pet shop to just let them sort it out for themselves and they will get over it once the older erts her dominance but it has been going on for hours. is it normal for this to happen?