Exercise 1: Draw mirror images of simple shapes such as a letter, number or basic shape. A few things to watch out for:
1. Make sure the mirror is partly overlapped by the figure in front of it.
2. Make sure that the figure in the mirror is not completely visible (the figure is cut off by the edge of the mirror).
3. Make sure the heights are correct. If necessary, use assisting lines for the correct heights, so that the figure and mirror image are approximately the same size.
4. Slanted lines accentuate the reflective surface of the mirror (often used in cartoon drawings).
Exercise 2: Draw a human figure from the side with his/her reflection in the mirror. Important are assisting lines for the correct heights, in the example the red assisting line under the chin.
Exercise 3: When you draw the mirror in foreshortened perspective it’s easier to make the figure from the side look in the mirror. Note the assisting lines that were needed; it takes a bit of fitting and measuring to make the figures mirror-inverted. An extra edge gives the mirror a bit of thickness. A mirror frame is of course also possible. Draw the example, if you like with another head.
Here a pupil has made a variation on exercise 3. He really created a person. He used two horizontal assisting lines for the correct heights and later erased them. I have marked his assisting lines with red.
Here’s a vain and hypocritical devil holding a mirror while painting a halo for himself.
To draw a person from the back looking in the mirror you really need a photo, a live model or a drawn/painted example. Besides the fact that you are actually drawing two portraits in one (one from the back and one from the front), you also have to deal with the correct posture, proportions and depth. The 19th-century American painter James Wells Champney (image) probably used a black-and-white photo as an aid.
EXTRA: take a picture or find an example of someone from the back looking into the mirror and try to draw it. Start with a simplified representation of the composition (see example). You can always use (printing and) tracing to help yourself on your way. You can make your own painting from such a simplified representation.