Do it when you are free. Sit in a relaxed posture or you may even lie down on a couch. The fear is that if you do this, you will get so relaxed that you will fall asleep, which is, good for insomniacs but for those who want to attain higher levels of awareness, it is necessary to persist for enough long in a good posture which prevents them from dozing off.
The practice is very easy; it is so easy that you may wonder how it can be so effective, but believe me, Buddha attained his Buddha -hood by this practice (I mean there were so many things he did before doing it, but, ultimately, it was this practice which resulted in the full blooming of his flower of consciousness). All you have to do is to be aware of each and every ingoing and outgoing breath. It seems easy, but believe me, if you could continue to practice it just for fifteen minutes, without getting distracted, you will directly land into the heaven! If you manage to concentrate for half an hour, you will land into bliss, which is the stop very next to the heaven! The prize to have a trip to heaven and to the bliss and then to the eternal bliss is not much but let me warn you: “DO NOT FALL ASLEEP”: else you will land in the dream land and you will miss the flight to the heaven and bliss!
What you have to do is, to pay attention to breathing (yeah I am saying this second time!) and you have to be aware of breathing, just pay attention to how your lungs contract and expand, how air touches your nostrils and how inhalation gets converted into exhalation. There is a certain point where inhalation gets converted into exhalation and that point is very critical for awareness. Pay close attention to the breathing and realize that you are not supposed to alter the way you breathe. You do not have to change anything but just to be aware of ‘what is’. It is true that your paying attention to the breathing alters the rate of breathing but it is not the same as you trying to alter it. Similarly, the depth of breathing will also change and your entire body will become relaxed, calm and cool. Your thoughts start slowing down and a point comes (if you persist enough long) when there are no thoughts and it is bliss!