Try gently pulling her leg out in front while holding her little foot so her leg is parallel to the ground. Brush the hair down towards the ground so it is hanging and using thinning shears to trim them. This will take it down and blend at the same time.
I don't use clippers on Yorkie legs if they have the proper silky hair. It just chops them up!
A good rule of thumb is always have shears lined up with the way the hair is growing. So if the hair grows down on her legs, never trim across it perpendicular.