Left Hand for Violin/Viola
Here are some pictures of what to look for in the left hand.
There should be a straight line from the back of your hand all the way to your elbow. Keep a straight wrist!
Something useful to watch for is the crease at the bottom of your first finger (index finger). If you line up that crease with the top of the fingerboard, your fingers will have an easier time falling on the tapes.