A lot of people tend not to wait for the club to complete the backswing before they start coming down. This means the upper body starts moving too early in the downswing, before the lower body.
This is the opposite of what it should be and often causes amateur golfers to come over the top, so the club gets outside the line and they hit across the ball with an out-to-in swing path. This causes a slice if your clubface is open to your swing path or a pull if it’s square or closed to the path. This is a good drill to stop your upper body from moving too soon and causing you to swing over the top.
Try this feet together drill
If you swing with your feet close together it will encourage your upper body to stay more still and your arms to drop a little rather than coming over the top at the start of the downswing so you can attack the ball more from the inside on a neutral plane.
Upper body goes first
When the upper body moves first in the downswing the right shoulder starts turning back towards the target before the hips and lower body do – this throws the club outside the line so you have to hit across the ball.
Lower body goes first
When the downswing starts from the ground up, as it should, then the lower half has already started turning back towards the target when the upper body begins to turn so the club drops into the slot and swings down on a neutral plane.