The technique is simple, breathe in through your nose for 4 seconds, hold for 7, breathe out slowly through your mouth for 8. Doing that a few times actually slows your heart rate down, that's not a feeling, it's a measurable effect.
A long exhale directly activates the nervous system that controls your body's calm-down signal. When the out-breath is kept longer than the in-breath, your body shifts into rest mode almost automatically.
It can feel awkward the first time, keeping track of the count takes some focus. But after a few nights your body starts to memorize the rhythm, and it takes less conscious effort to get the same effect. Doing it somewhere quiet, or with a soft sound in the background, makes it easier to stay with the count.