First, establish a stable posture. You may sit on a cushion on the floor or in a chair. Your legs may be crossed or in a kneeling position with your feet behind you. However you sit, the most important part is to keep your back vertical and straight, but relaxed, not rigid.
Think of your back as a stack of coins with your head balanced on top. Think of the crown of your head being suspended by a string from the ceiling, your chin slightly tucked. Your shoulders are relaxed. Your arms drop down from your shoulders in line with your ears.
Your hands may rest on your thighs or they may be cupped in your lap, left inside right, with the tips of your thumbs gently touching. Your eyes may be closed or slightly open with your gaze directed about three feet in front of you on the floor. Your mouth is closed; the tip of your tongue rests against the palate just behind your front teeth.
When you are comfortable with your posture, take three slow deep breaths. You are breathing from your diaphragm. Your abdomen rises and falls with your breathing. Your chest doesn’t move much.
After the first three deep breaths, breathe naturally, in and out, through your nose. Your breath should be quiet, very little or no sound. Feel the sensation of the breath as it passes the opening of your nostrils and across your upper lip. You may notice the coolness of the air coming in and the warmth of the air leaving your body. Focus on the sensation of the breath at your nostrils, or you may find it easier to focus on the rise and fall of lower abdomen with each breath. When the mind wanders, as it will, notice that and return to the anchor of the breath. Over and over.