Summarize this article:

Most people naturally have an overbite, which means their lips are not lined up on top of each other. Moving your jaw forward situates your bottom lip directly under your top lip. Be sure not to push your jaw too far forward. The right spot aligns your top and bottom teeth straight up and down.  Practice moving your jaw forward and backward while you buzz your lips to hear how this affects the buzz. You can also feel how it directs the airflow either down to your chin or up to your nose. Aligning your teeth directs the airflow straight forward into the mouthpiece for maximum airflow. Do not clamp your teeth together as this will hinder the airflow. Your teeth should be at least ¼” (6 mm) and at most ½” (12 mm) apart. Your jaw should feel like it is hanging loosely. Get the feel for this placement by putting the narrow end of the mouthpiece between your teeth. You can also use a pen or pencil, or a straw, which can help you test your airflow. Proper embouchure allows just the right amount of air to pass through your lips. If your tongue is on the roof of your mouth or moving around your mouth, it will block the airflow. Your tongue will move and tap your lips as you play the notes, but when you aren't tonguing notes, it should rest low in your mouth.

Summary:
Push your jaw forward to line up your lips. Leave about ¼” (6 mm) between your teeth. Keep your tongue low in your mouth.