There are two different ways I usually draw teeth: one is square teeth and the other is bell-shaped teeth. The only time you really want to use black lines to define individual teeth is when you draw someone who may have had gaps in their teeth or extremely crooked teeth. Many beginners and advanced students find it very difficult to draw realistic teeth in their portraits. Add 2 diagonal lines on each side and 2 curves about a third or a quarter of the way up, depending on where you want to position the top and bottom teeth.
You will also see in the video that we will use these basic teeth over and over again in the examples below by pulling the lips over the teeth in different positions and then either deleting or showing the teeth when opening the lips.