On several occasions, I've been asked by my students exactly how to visualize something while meditating. This is an interesting question. As an example, let's take a very basic grounding meditation, one where the practitioner visualizes their taiji pole glowing a golden color. One question I've gotten in the past is, "From which perspective should we view the taiji pole?" Should it be viewed from above, from the position of your eyes? From outside of your body looking at yourself? From inside the taiji pole, looking outward (or upward, or downward)? This was a difficult question for me to answer at first, because I had trouble recalling the perspective from which I personally viewed my taiji pole.
Eventually the answer came to me, however. I don't view it localized from a single space. It is viewed from any and all possible angles, simultaneously. While doing the meditation, you exist in all place and view the object of your visualization from any angle you wish, any angle that is necessary at that moment.
But this goes further - you also do not view it from a particular point in time. If the visualization is something that changes in time (such as a light descending or rising), you view it from all times. Before it starts, while it progresses, after it is finished, or anything in between.
Keep in mind that this is not always appropriate to every single type of meditation. But for the most part, when meditating, time and space do not matter.