The 3 Fundamental Principles most teaching professionals start with are GPA , Grip, Posture and Alignment. Then we move on to contact . You can have all three fundamentals wrong and have great consistent contact and therefore play great golf! (Simplistically put, if you can't hit a golf ball you cannot play golf).
You need to hit the golf ball with your irons where the lowest point of your swing arc is in front of the ball to some degree, thus having that "downward blow" you always hear about.
SOME of the things that can create a change in arc contact point with the golf ball:
A change in knee flex
A change in lead shoulder distance from the ground
A change in lead shoulder rotation
A change in spine angle
A change in lead arm flex
A change in release/amount of shaft lean aft impact compared to address.
The simplest 'DRILL' to create that point of contact in front of the ball that I have done with countless students is to put a tee or piece of string in front of the ball and have them hit the ball with their real target being the object in front of the ball. That minor change in perception of where they are aiming has always worked.
Listen, you can still hit a thin or fat shot, but guess what. We all hit bad shots. The key is to increase consistent contact and the best place to do this is in chipping in the short game area. As every student of mine has heard me say, Short Game helps Long game, not vice versa, practicing chipping will help with contact which improves how you hit the golf ball long with everything, including your driver. Poor contact will mean you can never have a consistent result and who wants that?
Hope this helps, and as always play well!