First as we become aware of how everything we are experiencing is simply a flow of events, coming and going, this slowly percolates through the system and changes our relationship to beings and things. As the Buddha says: nothing in the world is worth holding on to. Why? Because nothing remains anyway.
This is how clearly perceiving impermanence undermines the dukkha - suffering and unsatisfactoriness of attachment.
However, in Mahasi vipassana we are asked to look more closely, more minutely. This is why we make the great effort to slow down. Slowing down the body, slows the mind. Stopping and noting intention brings us back into an awareness of momentary moments.
As this gathers in focus, time collapses into minute moments of arising and passing away. We are not seeing the world out there arising and passing away, but individual acts of cognition. The amazing thing is that when a moment of cognition comes to an end, still there is awareness.
This is how we come to realise that we are not acts of cognition, which include all the khanda. The Aggregates of our physicality, perceptions, feelings, mental states and acts of cognition are not me, not mine, not a substantial self.
This is how by perceiving impermanence helps us to realise anatta- Not-self.
The corollary is to experience what we are – that awareness. Awareness is the deathless.
This is momentary dropping of the fetters of ignorance and delusive desire. A moment, like a flash of lightening into Nibbana.
So it is through the observation of impermanence that both suffering/unsatisfactoriness and anatta not-self are comprehended. But more, we come to realise that awareness, sati, is itself the One Who Knows, Buddho!
How can we bring this sort of observation into ordinary daily life? Very simple. Whatever you are doing, when it finishes, STOP! Acknowledge that that action of thought, speech or deed has come to an end and will never happen again.
It may be that the emotional attachments begin to rise there and then – nostalgia, grief, disappointment and so on. If we can wait for these to pass, all well and good. Otherwise recall the incident at the evening meditation and let them burn out there.
In this way, in ordinary daily life, we become aware of impermanence, of how attachment causes suffering and in allowing it all to rise and pass away, we realise it was all not me, not mine.