Take a position where your spine is erect but relaxed. If you are sitting in a chair, have your feet flat on the floor, your hands relaxed in your lap. Let your eyes close gently.
Bring your attention into your body.
Become aware of your body sitting. Feel your buttocks on the chair or cushion, feel the heaviness or lightness of your body. Notice the different contact points ... feet on the floor, hands in your lap. See if you can refine your awareness down to the point where you can feel your eyelashes on your cheek.
Now become aware of your body breathing. Notice that your body breathes all by itself. You do not need to control your breathing, just let your breath be natural. Notice the movement of the breath at the point that it is most noticeable, either the rise and fall of your belly, or the in and out of the air at the tip of your nose and upper lip.
Breath, and stay with it from its very beginning through to the end, then on to the next breath.
You may want to make a mental note in your mind, either "in and out" or "rising and falling", but the note should be only 10%. The actual sensation should be about 90%.
When your mind wanders away, which it will, when you become aware that you have been thinking, make a mental note of that, "thinking, thinking", and then bring your attention back to the breath. No judging the fact that your mind has wandered, just simply bring it back and start again. Just let go of the thinking and, with gratitude, bring it back to the breath.