If you struggle to swing on the correct plane, this is a great practice drill for you.
Take your stand bag, or your bag on a trolley and place it so that the end of the bag is on your target line - the end of the bag is on a straight line with the ball and the target. Put the end of the bag approximately five feet away from the ball to the right of the ball if you were set up ready to hit a golf shot. Set the stand out so that the bag angles away from the target line towards the side of the target line that you set up on.
Set up to hit your golf shot. Swing the club head along the target line towards the end of the golf bag, by turning from your left shoulder. As the club head approaches the golf bag, begin to hinge your wrist so that the club head moves upwards and just above the golf bag, make sure that the club head does not move underneath the bag as you are looking at it. Allow the golf head to move up the angle of the golf bag, just above the bag as you are looking at it. The golf club should then become parallel to the angle of the stand bag but your left arm should be still pointing just above the angle of the golf bag. Finally, continue rotating your shoulders 90 degrees to the right of where they started and now your left arm should appear parallel and just above the golf bag.
If you allow the club head or your left arm to drop below the upper edge of the stand bag, your swing has become flat. If the club head rises too high above the bag this results in the club shaft looking more vertical than the upper edge angle of the stand bag. If your left arm becomes more vertical than the angle of the stand bag, your swing has become too upright.
With practice you will be able to replicate the movement and hit more accurately, consistently and longer golf shots.