A flatter swing path is the one of the most common causes of a hooked golf shot.
I see flat swings quite often, most frequently in baseball and hockey players as these motion mimic the golf swing but in a more horizontal plane. To get away from this tendency you will have to make your swing more vertical, or in golfing lingo, upright.
As seen in the picture below, imagine a brick wall a couple of inches from your butt, or better yet find a real wall. With or without a club setup, and go through your back swing motion slowly. If your swing is too flat your hands or club will make contact with the wall, telling you you’re swing is still too flat. Working on making your swing vertical enough so you can avoid contacting the wall, this will ensure you downswing is on plane.