Start by developing you physical senses. Train yourself to be more observent, to notice more when you are looking, listening, and feeling. Lower your threshold of observation. Remember, your astral senses are just your physical senses on an astral level.
Once you have done that, start practicing at seeing auras. It just requires practice. Practice with someone standing in front of a solid, light color wall. Relax your eyes, like you are looking at something far away, and the other person is just in the way. Look mainly around the head and shoulders. You'll notice the Etheric Double first, the thin, light blue outline surrounding the body. Keep practicing, and soon you'll be seeing colors beyond that.
Remember, it just takes practice.