Most golfers hit slices, pulls and pull hooks because their downswings are too steep. Golfers with a steep downswing bring the club down with the shaft almost perpendicular to the ground.
The club is moving from outside the target line and cuts across at impact. In most cases, this also creates a chicken wing look. If you think this describes your swing, you can fix it and straighten out your shots by shallowing out your downswing and getting the shaft angle flatter on the downswing so the clubhead approaches the ball from inside the target line at impact.
This is a half-swing drill to get the feel of shallowing your club on your downswing.
1. Stand taller. A more upright posture makes it easier to turn the shoulder during your back-swing.
2. At the start of your swing keep the club further in front of your body and closer to the tar-get line. Players with steep downswings tend to pull the club inside in a more flat plane and loop it steeply outside during their downswing.
3. Halfway back, when your lead hand is parallel to the ground, point the grip of the club at a 90-degree angle down towards the ground. This is a drill to get the feel of shallowing the club later in your downswing. On an actual swing the shaft will be pointing down between your toes and the golf ball, exactly where is dependent on the posture and range of motion of each golfer.
4. When you start your downswing by shifting your weight to the lead foot and turning your hips towards the target, make sure your right shoulder, arms, and grip move down towards the ground, just for a brief moment, before the back end of the grip changes direction from grip pointing to the ground, to grip pointing towards the target line.
After practicing this drill you will be able to feel the shallowing of the club on the way down. Once you do, then you can try hitting a ball with the feel of an in to out path. At the beginning, you will probably not be able to strike the ball solidly, but keep practicing until you are able to strike the ball solidly and then apply this to your full swing.