The Dumbbell Goblet Squat is a great exercise to reinforce proper squat mechanics.
Start by holding the dumbbell (kettlebell can also be used) in the goblet position and placing the dumbbell against the chest. Stand with feet just beyond shoulder width apart and with natural foot position. Squat straight down until elbow comes near to or in contact with top of thighs, and rise straight up again to the start position. Strong exhale when driving upwards from the bottom position. The focus should be to lower and rise in a straight line.