To practice this exercise, deliberately stop everything else you are doing, and give your full attention to your breath.
Make sure your body is in a position that allows for easy breathing—either sitting upright or lying flat on your back. If you feel comfortable, close your eyes. See if you can bring all of your attention to the present moment of breathing.
Pay attention to the sensation of breathing in and out, focusing on any spot in the body where the breathing is easy to notice. This could be at the nostrils, chest or at the abdomen. Stay with that spot, noticing how it feels as you breathe in and out. One breath at a time. You don't have to force the breath, or bear down too heavily with your focus. Let the breath flow naturally, and simply keep track of how the present breath feels.
You are going to be practicing 20 very short mindfulness practices. Each practice only lasts for the duration of one full in-and-out breath, about 5 or 10 seconds. Every breath is different. Each one is short enough for you to devote yourself to it fully. Don’t worry about what happened during the last breath, or what will come next. For the first inhale and exhale, count “1,” and continue counting up to “10.” Then count backwards to “0.” Don’t judge distractions, just notice them. When your mind wanders off, or if you lose count, that is okay, it's not a problem.
Simply guess or choose a number, and bring your attention back to your breathing. Notice the distractions; you don't even need to resist them. Once you notice them, you can label them (thinking, wandering...) and return to the breath.
Wrap Up Slowly and Gently. At the end of counting, pause to sense the body noticing the world around you.
Adapted from: http://www.accesstoinsight.org/lib/authors/thanissaro/breathmed.html and http://www.pennmedicine.org/stress/