I love the goblet squat. It's a great entry way into teaching people correct squat form. One of the questions I get about goblet squatting is regarding leg and foot placement. Meaning, should I be in a narrow or wider stance?
Using the body rock technique above, you can determine the width of your stance that allows you to keep a back flat through the movement. To do this you can either have someone observe you or video it with your phone.
Start on all fours with your hands under your shoulders, knees under your hips and ankles dorsiflexed. Put your stance in a hip width or narrow position.
Push you butt back towards your heels while trying to maintain a flat back. The distance you are shooting for is the knees behind the hip crease. If you are unable to keep your back flat through this range of motion in a hip width stance, slightly widen your stance and try again. Eventually you find a stance that allow you to keep your back flat through this range of motion. When you find this, this is the stance you will use in your goblet squat.