Breath is the bridge between body and mind. Sit upright, close your eyes, and simply notice inhale and exhale without trying to change them.
When the mind wanders, gently return to the sensation of air at the nostrils. After two or three minutes, lengthen the exhale: count four in, six to eight out.
This activates the parasympathetic nervous system and helps before sleep or a tough conversation. Five minutes daily is enough for a noticeable effect.