It's important to understand how one element of technique depends upon another. For example, the left wrist puts the hand here or there, in this position or that. If you forget that simple concept, you'll work harder than you have to. Let's go over the "hierarchy" of technique to appreciate how interdependent different parts of your body are to smooth technique:
1.The music calls for certain locations on the fretboard.
2.The fingers go to the locations.
3.The left hand puts the fingers there.
4.The left wrist puts the left hand there.
5.The left forearm puts the left wrist there.
6.The left elbow puts the left forearm there.
7.The upper left arm puts the left elbow there.
8.The left shoulder puts the upper left arm there.
9.The torso puts the left shoulder there.
10. The sitting position places the torso there.
Now try some familiar chords with these thoughts in mind. See how useful a change of arm and wrist position is when switching through chords like B7, G7, E, D, D9, etc.
Note: The angle of the guitar neck (part of the sitting position) greatly influences the success of the left hand. See Sitting for more on this.
Left hand thumb position and barre technique and tips are covered elsewhere on this site.