Standing with feet slightly farther than shoulder width apart, sit down like you are taking a seat on a low bench. Hold your hands wherever they feel comfortable. I like to hold my arms in front of me at shoulder height; hands or fingers clasped. Really stick your rear end out. Trust me, you won’t fall down backwards. Take a look at your knees and feet. Your knees should not extend past your toes. This is where I see the most errors. Extending your knees beyond your toes puts extra pressure and stress on your knees.

Performing squats incorrectly over an extended period of time will lead to knee problems and joint damage.
To reinforce this tip, sometimes in my classes, I will have a “contest” with my students to see who can stick their butts out the farthest. Keeping your posterior poked out will help to ensure that you maintain proper form.