One of the major differences between a high handicapper or inexperienced golfer and a better player or professional, is their ability to understand and control their swing path.
Generally speaking, better players and professionals can control their swing path in order to hit the shot shapes that they desire. Whereas less experienced golfers and higher handicappers have a tendency to either misunderstand their flight, or be less able to make the required alterations.
Many better players and golf instructors would prescribe that an inside-outside swing path is a preferred path to that of an outside-inside path. This is certainly applicable for most of the longer golf clubs, as it will generally produce a longer or penetrating and slightly drawing ball flight.
Conversely, an out to in swing path is often described as steeper and would normally produce a left to right flight, which could result in shorter slicing golf shots.
In order to create an inside-outside swing path, focus on bringing the golf club down more behind your body in the initial phase of your downswing, keeping your hands relatively close to your rear leg to the bottom part of your downswing and extending your arms out in front of you.
In the follow-through motion, the arms should feel like they're extending to the right of the target line for the right-handed golfer and left for the left handed golfer.
A good drill to help focus on creating the right swing path would be to take two golf clubs, place one on the floor indicating your ball to target line and stand parallel to this club. Now take the second club and point it right of the target line for the right-handed golfer by about 15° and now practice making your swing follow the second golf club so it follows on an in to out path. This is the first element of trying to create stronger more penetrating longer draw shots.