Cutting Dog Nails

I just have never felt comfortable cutting my dogs nails. I know that if you cut them too far back that you could really hurt them so I am always scared to do it wrong. I have to bring Gracie to the groomer because her nails are just so incredibly long. I don't know if it her diet or if they just forgot to do it last time but her nails are really long and I can hear them when she walks on the floor. It must really bother her so I will just get them clipped a bit.