Look beside the head - 2-6 inches to the left or the right and defocus your eyes. Move the point of focus a few inches further away sideways and then back and forth, slowly. Experiment. Because everybody is different, nobody will be able to explain to you exactly how to do it. You are looking for a change in the light level around the person's head. There is a band of mostly white light 2 to 4 inches around people as the first layer of the aura, the etheric body. This is the easiest to see and the other layers extend out like layers of an onion. It is important to remain detached and objective. The trick is to be able to accept what you are seeing and not dismiss it as something you are making up.
You can also place a heater in front of the wall and try to see the heat waves, or try to see the wind (not the effect of the wind) blowing outside.
Don't get anxious if what you see is different, as everyone's experiences differ somewhat. Likewise, it is probable that it will take you quite a few times of practice before you begin to gain any results. If you are dedicated, determined and patient, you will see auras.
Seeing auras works best with high energy focused people, as they have the brightest energy radiation and are crisper in the arrangement of their patterns. The aura is a more complete description of a person. It contains information concerning the entirety of that being. Understanding this information can be difficult as it is not necesarily in a format which we understand. I think that it is best to combine an instinctive approach with empirical methods when interpreting energy forms viewed. The colours of the aura correspond in meaning the the colours of the chakras, but this is not a complete interpretation.
Experience is the best teacher.