When you chant Shiva mantra, your body should be still. Your spine, neck and head should be erect and your body should not be moving at all. You can begin by whispering the chant. Mantra is a sound vibration which will bring alterations in your brain and in the energy field of your body. By chanting Om Namaha Shivaya, you are not pleasing Lord Shiva but you are using this particular sound wave to create a state of mind where there is no vikshepa (disturbance of mind). That is why it should begin with the whispering notes. Your focus of attention should be the mid eyebrow, tip of the nose and your face. Your own head is the real Shivalinga. The total focus should be there and this means that the rest of the body doesn’t exist for you, your focus of attention should be this sharp.
The second way is by doing it in your breath. When you take one long deep breath, initially you might be able to do the mantra only once, and when you exhale, in one exhalation you do one mantra. But by regular practice your breath will get longer and deeper, and soon you will be able to do two, three, four or five mantras in one breath. And as your mind gets more and more focused and internalized, the third stage will automatically come, when you don’t have to say the mantra in your mind anymore, but the mantra takes over and this vibration begins to pulsate in your body, in your breath, and it may take you to a higher state where the mind stops, and at that state the mantra chant will also stop. This third stage is the most wonderful spiritual experience.