Your golf swing is three-dimensional. It has height, width and depth. If you understand how each of the three dimensions works separately, you can properly blend them together.
1. Get a vertical hinge
Your hands and wrists hinge the club up and down relative to the ball. To practice the height dimension, hold your 5-iron in front of your chest so the shaft is perpendicular to the ground.
Now, raise the clubhead over your head. Feel how your wrists hinge the club upward, and your biceps lift it higher? That's how you should lever the club up on the backswing. Now move it back down your hands and wrists unhinge. That motion should occur in your forward swing.
2. Make a jab
Your left arm determines your swing width. To feel the proper width, take your address position without a club, turn your shoulders back and throw a left jab at the air to your right, extending your left arm as far as it can comfortably go. On your forward swing, extend your right arm.
3. Lead with your forearms
Your right forearm creates depth as it pulls your hips and shoulders into a full backswing. That movement needs to be fairly level: If you move your right elbow up and down, you won't get the depth you need to make a full backswing turn and you'll likely mis-hit the ball.
To practice this movement, take a 5-iron and swing to a three-quarters backswing. Your right elbow should be over your right hip at about the same height it was at address. It's also as deep as you want your swing to get. From here, you have a direct path to the end of your backswing, as your wrists hinge the club up and your left arm provides the width you need to complete the three-dimensional effect and make solid contact.