The first thing to go over is appropriate trunk posture, not foot position or stance breadth. Assume you're a soldier, and the meanest, ugliest sergeant in history has just informed you, "TEN-HUT!" You'd naturally straighten up and draw your head and shoulders back. This is the appropriate spine posture for squatting. That is, your head is drawn back, your chest is elevated, and your lower back has a small arch. At no point throughout the squat should you lean over or gaze down. Of course, you must bend over at the hips (more on that later). You should also avoid looking up. So you've got it down?
Now, the easiest method to perform squats is in a power rack or cage (a huge rectangular rack with cross-drilled holes) so you can change the pins and lay the bar down safely if you have to stop. Set the pins to slightly below the desired depth. They also act as a visual indicator for depth and whether you're going down/up the wrong way. Place the J hooks or posts that hold the bar for you to go beneath near your nipple. Unrack it once to check whether it's at the proper height. If the bar lacks a knurled section in the center, choose another bar or gym.
https://www.youtube.com/watch?v=9XXpN3Ac_m8