"Zen meditation" sounds like something special, but most likely you've already done it. Have you ever sat down to take a break, just be yourself, and let yourself "heal to wholeness"? Zen meditation is the same thing, but with the determination and commitment to stay with it, to let it naturally become stronger and deeper. The following are things that you'd probably realize on your own sooner or later as your practice of it matures. They are the main things that seem to have helped most people with it over the centuries. And so some of them will probably help you now and then, as needed. But none of them are intended to take the place of your own experience of meditation. That is a much better guide, more intimate and sensitive.
Actually the word Zen means, roughly, meditation. The word for Zen meditation is zazen, where the za means to sit. There are three aspects to this: posture, breathing and calming your mind.
Positions
The two "families" of postures are the kneeling positions and the lotus postures. Kneeling includes sitting in a chair, with a small cushion behind the small of your back so that avoiding slumping is effortless. Then there's the kneeling bench (on a zabuton, a flat rectangular cushion), with which there is zero pressure on your ankles and little pressure on your knees. And you could kneel on the zabuton with a soft round cushion (a zafu) between your seat and your lower legs.
The lotus postures include quarter-lotus, Burmese, half-lotus and full lotus. These are better seen than read about, and better tried out -- felt -- than seen. In these, you are seated on a zafu on a zabuton with your legs on the zabuton too. The point is that your seat and the outsides of your knees form a very stable tripod. And if they don't (because, say, you're not loose enough in the legs), you can choose a bigger zafu (or zafus) to sit on, and even put a small cushion under one knee, to make sure that you're firmly "planted".
Just as what's cooking and how the cooking is going is what's important in cooking rather than which pot you're cooking in, all these postures are just to get yourself "in a position" for deep zazen. They are not ends in themselves. What you need is to be alert and at the same time relaxed. For this you need to sit up straight, or maybe I should say, "relax up straight". And it helps if you're close to the floor, "well grounded". No posture is going to give you all of this. But the ones above come, in different ways, close.
The kneeling postures using the bench or a zafu almost automatically "erect your spine" so that you feel a slight pressure low in the small of your back, like a snake sitting on its tail. In the other postures you can do this yourself, as needed. By "erect your spine," I don't mean "sit up straight (!)" using a lot of muscle, belly in, chest out. It's more like: with belly relaxed out a bit, chest naturally not pushed out, "relax up straight": the back of your neck relaxed so that your head tilts very slightly forward, and then so on down your spine, subtly relaxing so that the space between each pair of bumps in the middle of your back are very slightly more apart. It's as if a cord attached to the top of your head were pulling gently upward.
No position is entirely comfortable until your body has a chance to settle in. Sitting with others helps with this: the gentle pressure to sit still so as not to disturb them helps you sit still long enough that your body can loosen up a bit and settle in.
If you happened to be disturbed about something, it's good to sit with your eyes closed to let your body deal with the inner turmoil. But normally sit with your eyes open -- to avoid your zazen becoming a separate place to hide from the rest of your life. With eyes open as they are in your days, there's no reason why the deep peace of mind and alertness you're cultivating in zazen can't carry over on out into your everyday life.
The Soto Zen school uses the "perfection" hand position: the hands form a vertical circle, thumb-to-thumb, fingers of one hand over those of the other in your lap. We of the Rinzai Zen school favor the "determination" position: right hand grasping left thumb, left fingers around right hand, and the result in your lap. Either way, your hands being near your center of gravity "concentrates" your body―which gently, subtly, both helps your mind concentrate and helps you "heal to wholeness".
Calming Your Mind With Your Breath
Zazen is actually quite natural. If you sit still, that is, to the extent that you sit still, your breath calms down, slows down. Nothing is wrong, but the mind (and we're all different in this) tends to worry about your rate of breathing, blow up the discomfort (associated with your position until your body settles in) into something serious, and, on top of that, to play a kind of emotionally involving video featuring your nostalgia and regrets about the past, hopes and dreams about the future, obsessing on whatever has affected it recently -- and generally anything it can come up with to avoid giving up its starring role by getting you moving and taking in a swift stream of sensory impressions.
The way to calm your mind that has stood the test of time is counting your breaths, silently, on the out-breath, from one to ten, not worrying about making it to ten. If you lose count, just go back to one again. Breathing is wholly familiar, so putting your mind on it will be just enough to keep that mind from running around beside itself. And if and when that gets to be "too much," follow your breath -- that is, count without the numbers. And whenever you later find yourself getting tired of thinking about something, just use that chance: Let go of it.
But the only essential ingredients are your own courage and determination. Those things discussed above don't have to be mastered to start sitting. They are just pointers to get you started, for occasional use, as needed. But beware of letting any of them become a crutch. They are not intended to take the place of your own experience of zazen. That is a much better guide.