Drawing people's reflections in mirrors is as easy as drawing the people themselves. All you need to do is practice symmetry and perspective to get the right look in your drawing. These two processes are combined on your paper to resemble mirror images, when all you are really doing is simply drawing the person at a different angle.
Draw the person the mirror is going to reflect first. This is essential for getting your angles right the first time.
Draw a straight line from the top of the person's head to the mirror. Wherever the line lands on the mirror will be where you will start drawing the top of your reflection.
Decide which part of the head will be reflected in the mirror. If the person has her head turned away from the mirror, the mirror would reflect the back of her head, for example. A good rule of thumb is to pretend you are the mirror. Imagine standing where the mirror is and looking at the person. What you would see is what the mirror would reflect.
Draw the reflection smaller if the mirror is far away from the person. If the mirror is right beside the person, draw the reflection in the same proportions.
Erase your guideline you drew in step two.