Be seated on the tip a chair or on a cushion on the floor.
Keep your legs in a position that you are able to maintain comfortably for a length of time. In the position, keep one leg on the other thigh.
In a complete lotus position, keep both feet on opposite thighs. You might even have to sit simply with your legs tucked in close to your body, if you can't manage the other way. Then again make sure that your body weight is dealt out on 3 points.
These are both your knees on the ground and your buttocks on the round cushion. On a chair, you should place your knees at a distance around shoulder width, feet kept firmly on the ground.
Sit straight and stretch your spine, Keep it naturally erect. Center your balance in your lower abdomen. Thrust your lower back somewhat forwards, open up the chest, and slip your chin inside a little. Keeping your head straight, not leaning forward, or backwards, nor to the side. Bend your body gently from side to side in diminishing arcs. Do this till you automatically reach a point of stillness on your seat.
Taking a deep inhalation, then exhale completely. Now take another deep breath, then again exhale totally and completely. Leave your breath to settle to its natural rhythm and with the right pose (asana), your breathing will flow naturally to your lower abdomen.
Breathe naturally, without judgment. Nor should you try to breathe a particular manner.
Keep focusing on your breathing. Should your attention wanders, take it back to your breath time and again. Do these as many times as you find you have to! Stay still, as much as possible, following your breath and coming to it whenever thoughts come to mind. Be completely, crucially and fundamentally present to the moment and to yourself. Just do your best. At the end of your period of sitting, quietly and slowly sway your body from side in rising arcs. Stretch your legs, and make sure that they have feeling in them before you stand.
Do this on a daily basis for at least 10 to 20 minutes (longer if you can manage) and you will find the treasure house of the timeless life of Zen - your very own Self.