I've heard bad things about PetSmart groomers-so definitely search for another one
I agree, just tell them what you want, or more importantly, what you DON'T want. If you can find a picture of a yorkie that has a haircut that you like, bring it with you, or do a sketch with arrows and very descriptive wording
I always tell them to do the following:
-Trim up feet-sides and pads
-Shave lower belly and clean up "potty" areas
-Shave top half of ears, inside and out
-Do NOT shave down bridge of nose
-Do NOT cut hair on top of their heads (long for ponytails)
-Leave beard long, but clean up
-Trim up body
Hope that helps get you started-take any of these and modify as you like!