Your backswing plane should be more upright than the down swing plane. Unfortunately a lot of golfers' swing plane is the exact opposite of this and it turns into swinging over the top.
Swinging over the top is one of the biggest causes of the ball starting to the left and turning into a slice or a pull.
Giving you the upright backswing drill has already started to cure the over-the-top problem. But now we need to work on flattening your downswing plane. But before I give you a drill that will help you to do that I have a word of warning.