You should probably have someone else show you before you try to do it yourself. As far as I know, teeth in general do have nerves at the roots, but you shouldn't get anywhere near them. I watched a vet tech at my work trim mouse teeth just yesterday - she said you need to leave enough of the tooth so that they can still use them & so you don't risk getting near the root, but again, something that would be good to have someone experienced show you. Also she mentioned that with rats & rabbits, you need to use a dremel, you can't clip them. I'm guessing with gerbils, it'd be fine to clip as they're smaller, but something else to double check on.
Can you see why they're overgrown? If she doesn't have enough to chew on, that could be the issue. Is there any chance the other might be hogging chewing toys or something, since that one has no issues? Otherwise it sounds like she needs a vet visit even besides the trimming - her teeth could be misaligned, making it difficult for her to chew properly & that will require regular trimming for the rest of her life. Or she could have something else going on that's making it hard for her to chew normally. In any of those cases, a vet visit is necessary.
~*~*~ Kelsey ~*~*~
RIP my sweet Lily ~ 6/12/08 - 1/20/12