To start listen to a noise you hear. This may be every noise around like the dropping of a tube, a distant road or whatever you hear. To practice try to listen to the sound as long as you can. This means as long as your mind stays concentrated on it.
This is all the exercise. After a while you will notice that you no longer stay focused on the sound. To improve your awareness of sound it is enough to this repeatedly. But maybe you find out what it is that turns your mind away from listening. And if you find out why you were no longer concentrated you notice a point in your life which disturbs you. Maybe there is something you can do about it.
An advanced version when your listening abilities are very good and you can concentrate a very long time on one sound then you might go on and concentrate on more than one sound at the same time.
Enjoy listening to your environment! If you like this exercise you maybe also find the last listening exercise on this blog useful: Back in Time.