Dear Kim and Minh,
I commend you for trying to work from life!
Doing a self-portrait is probably the easiest reference one can get in searching for the answers, particularly in figurative work.
I always say to my students, that the best model you can get is You. The model you see in the mirror can sit patiently for you as long as you need: What a treat!
Below is my multi-figured painting, where all the figures were created with the help of the mirror. Anatomic correctness of the positions of figures, facial expressions, positions of hands, all were done under the same light (from one side, approx. 45 degrees angle from above, which is the light's most "natural" position and makes any painting look "convincing" ). I simply posed myself, when I painted it, making those mimics, just drew different and imaginative faces, as we all have the same anatomic structure, although I've been told that some of them still look like me...
The mirror is certainly a great tool to practice your skills. It can be set up on any angle, according to your eye level. It would depend on your initial artistic goal, how you want the head to be seen: on your eye level, from below or even from above. You can also set up three mirrors, this way you can draw your own profile. Just make sure that you don't move the head ( only your eyes ) and draw from the same angle from start to completion.
I hope it was helpful, Good Luck and best wishes!