This is not an easy question. SA has no firm definition. For example, some people think it means to be extra vigilant and attuned to one’s surroundings. Others might say it means to not be hyperfocused on some aspect of the environment, but to be more broadly watchful to spot something out of the ordinary.
The first type of SA above is something you can consciously do, but the second type requires more use of your subconscious mind. Over and over, we hear of a victim saying, “something felt wrong, but I went ahead anyway…” And that “something,” whatever it was, was something out of place, or a lack of something that should have been there.
It is equally difficult to walk around with a magnifying glass in front of your eyes as it is to walk around looking through a telescope. Rather, you have to constantly zoom in and zoom out. I think good SA is kind of like that. You both focus hard when something triggers your attention, and you relax and try to be aware of everything going on around you, using all of your senses.
An exercise I practice occasionally in martial arts is to visualize what is going on behind me. This is a skill you can develop and it helps to build a 360-degree zone of awareness about your person.