Here's a link with pictures...I cut my girls right at the long end of the curve and I'm like you - scared to go too short
http://www.thepetcenter.com/gen/nailtrim.htm
this one is really easy to see where
http://www.planet-pets.com/clip_nails.htm
and the last pics on this site are great -
http://www.vetmed.wsu.edu/clientED/dog_nails.asp
they aren't yorkie nails but it's the same thing - good luck -