I personally haven't cut my girl's nails yet, since she's on sealed tile, and slate...they wear down fine just from her wandering. She's a little over six months, and has been on tile for 4 of those.
From what I understand, you should only be trimming the very tip of the nail, that is clear and beyond the dark stripe in the nail. To me it sounds similar to the cutting below the "quick" of a dog or cat's nails, except to a much smaller extent.
My common bearded has dark nails, but I see a very small section of clear beyond that. I don't bother clipping, but only because she wears them down naturally, so I've never even seen hers long enough to need trimming.
This video should probably help as well. Definitely give it a peek..
https://www.youtube.com/watch?v=dnK9DhOnOpU