Pay attention to this present moment, on purpose, and without judgment.
Mindfulness is the nonjudgmental awareness of the present moment. It is, at its core, a practice of attention training.
We begin by placing our attention on our breath. Just following our breath as it flows in and out. Perhaps connecting with the cool sensation in our nose as we breathe in. Whenever we notice that our attention has drifted off, we simply acknowledge it, let go of our thoughts, and return our attention to the breath. Breathing in, feeling the coolness in our nose. We do this over and over again, with compassion for ourselves.
Some may find it helpful to practice with a guided mindfulness meditation. I have a 10-minutes guided mindfulness meditation here.
Mindfulness FAQ 