You can't actually conceive of or experience true nothingness. Instead, you are always experiencing something as long as you exist and are conscious, so what you do is just constantly "push away" or "reject" all incoming information, all incoming experiences and sensations, all thoughts, all emotions, as "distractions", and you try to do this without falling asleep.
Right now, you probably imagine that you understand what I just described. Let me give you an example of this within a cultural context instead of just abstract concepts.
In my practice, I perform something called "utiseta" which means "sitting out" basically. "Seta" just means "sitting" or "to sit" or "seat". It is a form of meditation where you just sit there and try to enter what you might call 'satori' or "no-mind". You do this by finding a crossroads or a burial mound and sitting nearby or on top of it, and going "under the cloak", which involves laying on top of a black animal skin, and covering yourself with another black animal skin, so you are completely in the dark. In this state, you are holding a small hand axe and staring at the blade. Your eyes must not move left or right, no matter what you see, no matter what you hear, you must not respond. You begin first with a "vardloka" a song meant to invite the spirits for which the song was composed. If they hear it and are called by it, they will come to you and reveal themselves to you in some fashion. But again, you must not look, you must not move, you must not answer them, you must remain totally focused on the blade of the axe and your singing.
The hypnotic act of singing and the exotic situation is meant to occupy the mind with some tangential exercise that keeps it busy so that the rational ego cannot assert itself and dictate the narrative of what is being perceived. In this way, the unconscious is free to express itself, and it is through the unconscious that non-human intelligences (spirits, demons, ghosts, aliens, whatever you want to call them) can operate and "make contact" with us, but only when the conscious ego, the "self" is not interfering with the mental interface. You are quite literally allowing "something else" to take partial and temporary control over your entire reality, your body, your mind, in order to "commune" with you, in that place, for that moment. This cannot be accomplished if "you" are still gripping the steering wheel with white knuckles.
So the "true nothingness" I am trying to describe here, is not necessarily the non-existence of anything at all, but the non-existence of the sense of reality that ordinarily we maintain all the time in order to feel like we are in control of the narrative of our existence. The temporary "de-realization" that feels like a liminal state of psychosis, sometimes even feeling a bit like aphasia, is more like a state of receptivity to having an experience which you have never had before, and this is as close as it gets to "true nothingness". Because whatever novel experience we have, if it is truly novel, then it just was "true nothingness" as far as we are concerned and to us, up until the point that it enters our reality and "becomes" something that is not a mere repetition of the past. To be something truly different then it would have to be something which could be integrated into the whole, but appears to us in a differentiated state, and so we can only infer that it "exists", but we cannot prove it until we integrate it into some pre-existing framework for labeling it as such. Therefore, something completely different from everything else that has been integrated into our framework for whatever we believe existence to be, would necessarily appear to us as a "lack", something like Kurt Godel's famous "gap" of incompleteness built into any formal system complex enough to allow for compression of the information contained within it (i.e., capable of "godel numbering").