Shamatha or mindfulness meditation is a very organic practice, a foundational practice. It's based on noticing the moment when our awareness connects with our present situation, and actually deliberately cultivating that kind of simple awareness. The benefit is that we become more synchronized in body and mind and begin to relate to our world in a less distracted and more wakeful way.
To practice shamatha / mindfulness as a formal meditation practice, there are three steps:
1. Taking our seat
We start by taking our meditation seat, usually a solid cross-legged sitting position on a cushion on the floor. You'll want to get into a comfortable posture with a good firm connection between your butt and the cushion --- you should feel grounded and stable. Then you can just rest your hands on your thighs or your knees, depending on how long your arms are.
Your torso, head and shoulders should be upright but relaxed. Then just gently tuck your chin in a little bit. The posture should feel dignified and uplifted but not stiff or tense.
Of course if you have trouble sitting cross-legged for whatever reason, you can take a kneeling posture or just sit upright on a chair. The main point is to use whatever support you need (cushions, etc.) to be comfortable, but in any of these positions your back should be as straight as possible and not supported by the wall or the back of the chair. In general we say "not too tight and not too loose" and that is a good guideline all the way through.
There's a feeling of containment, of taking your seat and reducing your activity, your sphere of activity. Then, you can make sure your jaw is relaxed, either lightly closed or slightly open. Your eyes are open with a soft downward gaze, maybe four to six feet on the floor in front of you. You're not shutting down your awareness of the space around you, but you can relax your focus somewhat.
2. Placing Your Attention on Your Breathing
Having settled your body in this way you begin to pay attention to your breathing -- in and out. In this case it's natural breathing -- not pranayama or any other breathing technique -- just ordinary breathing. Your awareness becomes connected to your breath. Here again there is a light touch rather than becoming too intense and hyper-focused. Just a nice relaxed attentiveness to the breath going in and out of your body.
When you notice that your awareness is elsewhere -- maybe you're thinking of your relationship, your work week coming up, or a big bowl of chocolate ice cream. Whatever your thoughts are, when you notice that your mind is somewhere else, just bring your attention back to your breathing, without any kind of judgment or commenting or evaluation. Just bring it back.
3. Labeling thoughts as thinking
When you notice that you're thinking, just say to yourself, "thinking." Just label it with that one word, "thinking", and then bring your attention back once again to your breath. Trungpa Rinpoche used to say that when you are sitting like this, you have a flat bottom and your thoughts also have a flat bottom. Before maybe your thoughts had little wings and were flying all around and taking you with them, but now your body is settled and your mental activity will settle down as well.
It is also helpful to take a "democratic" approach toward our thoughts. They are all equal in some sense. We do not favor some thoughts or freak out about others. Very simply, when you notice you are thinking and have drifted away from awareness of the breath, just label any and all thoughts "thinking" and bring your attention back to your breath. It is important to note that we are not repressing our thoughts and neither are we following them. We are simply letting them be as they are, noticing them, and then returning our attention to our breath.
During longer periods of sitting -- if you need to move to restore your circulation, or when you have real discomfort, you can just bring your legs up in front of your chest and continue your practice. Then just resettle yourself and take a fresh start.
That's a basic introduction to the technique of shamatha / mindfulness meditation -- how to actually go about it. It's OK to start modestly at first, maybe sitting once every other day for 10-15 minutes or so. From there you can build up to a daily sitting practice for however long feels right for you. Once you get underway, working with a teacher can be very helpful. Meditating in a group setting can also be beneficial and can give you context, structure and support for your practice.
David Nichtern and Cyndi Lee's DVD, Om Yoga and Meditation is available through Amazon.com.