Fixing the "inside takeaway" can be a very difficult swing flaw to remedy. For many, letting the club roll to the inside going back feels very natural and easy. The problem is that it can lead to many issues coming down into impact. Most notably, the classic "over the top" move that plagues many amateur golfers' swings. There are a few keys that will help you get the club more on plane and less behind you for better club and face alignments at the top of your swing.
1.) Momentum- Feeling like you get a lot of participation from your core turning back in the takeaway will create some momentum, making it easier to keep the club outside your hands and the club more "on plane."
2.) Keep the wrists quiet until the hands are hip high- As you create this momentum with some good core movement, notice that the wrists haven't changed very much at all from their position at address to this hip high position. There may be some hinge (or set) in your wrists, but very little.
3.) Let the right wrist bend backwards- Once the club moves past hip high, instead of feeling the right hand roll under to where the right palm is facing the sky, feel like the right hand simply bends slightly backwards and the palm is facing more behind you or towards the ground.
4.) As you keep turning and complete your backswing your right hand will naturally rotate to a "server holding a tray" position at the top- Letting your right wrist bend backwards with your palm facing more towards the ground will feel awkward at first as you move the club past hip high, but as you continue to turn your right hand and arm will rotate to where the palm is facing the sky at the very top of the swing.
So instead of the palm facing the sky early in the backswing which gets the club inside early, the palm will feel like it only faces the sky at the very top of the swing or the very end of the backswing.