Position the ball against a wall, maintaining contact between the top of the ball and your tailbone, low back, and mid-back. Your feet should be shoulder-width or slightly more apart, and remain flat on the ground; walk your feet out far enough so your knees won't pass over your toes during the squat.
Keep the majority of your weight in your heels for the duration of the squat, making sure your knees face forward. Maintain pressure between your back and the ball the entirety of the exercise. Perform two to three sets of 12 to 15 repetitions.
Make it more challenging: Try raising your hands over your head when you're at the low point of your squat, or try doing split squats.