How to Improve Your Jawline with a Short Beard?
Something to keep in mind, and this is a common mistake made by inexperienced beardsmiths, is that most jaws are not perfectly symmetrical, but your beard can be. Don't follow your jawline in any way, shape, or form when cutting your beard. You are looking for even and level spots on your neck.
Symmetry can be achieved by looking up, taking a picture, choosing spots in the picture that look symmetrical, looking up again, combing those spots down to separate the hair that you are going to cut and then combing up the hair you want to keep. I like to leave it a little lower in the middle and a little higher on the sides. Whether you do this will depend on your chin shape and what looks best for your face. Take another picture. Ensure this looks suitable to your personal look, style, face, and build. Then carefully cut those hairs down a little at a time so that if you make a mistake you don't lose all of your progress.
Do this with sharp barber scissors. If you take ANY part of your beard down to the skin, you should learn to use a straight razor to get perfect lines. Any other kind of razor and you can't see what you're doing. Those razors are meant for shaving large areas, not creating fine lines.
Something else to keep in mind is that unless you are ambidextrous, you will be cutting at different angles from one side of your face to the other. This is why some men choose to have a barber make these cuts for them when they're first starting their beards. In the early days of beard growth, these small details matter more than someone who has a full beard and is just making adjustments.
The longer your beard, the lower you probably want your neckline to be. It gives fullness to a long beard by pushing out the frontmost layer of hair. I understand the impulse to reverse that logic and say that if you have an extra short beard you should follow your jawline. You shouldn't unless what you're going for is a pencil beard look. You're creating a new jawline, NOT following your natural one. This can give the illusion of a mighty jaw that will intimidate your enemies where without a beard you might go straight from mouth to Adam's apple.