You’re using the breath as an anchor for your attention, so that your attention doesn’t keep gravitating back over to the buzz of thoughts. Posture is important because it affects attention, particularly keeping the spine erect. If flexibility is a problem for you, just sit in a chair with some extra padding under your hips, so that your knees are lower than your hips. Don’t collapse back against the back of the chair; find the position of your torso where it feels balanced, both fore and aft and side to side. Reach toward the ceiling with the crown of your head. Keep your eyes open but oriented slightly downward. Fold you hands on your lap, against your belly. Your belly and chest should be out, your shoulders back and relaxed, your lower back slightly dished in, your head pulled back so it rests directly over the spine, and your mouth closed. Then relax as much of your musculature as you can without losing this position.
Now focus your attention on the breath. The counting is just a way of assuring that your attention hasn’t wandered too far from the breath. Breathe normally, and count each inhalation and exhalation until you get to ten, then begin again at one. You count silently, but let each count extend all the way through the inhalation or exhalation.
As you’re doing this, thoughts will continue to run through your head, and you attention will want to go to them and examine their contents. Your challenge is to be aware that the thoughts are there, but to keep the attention pinned to the breath. If you do this, the thoughts will simply arise and then leave. If you engage them though (as you will from time to time because that’s your habit), They will pull your attention away from the breath and lead it down a long succession of thoughts. You’ll know when this has happened because you will have lost the count. In this case, just start back at one with the next inhalation or exhalation.
If it ever gets easy for you to keep the count over, say, twenty minutes, increase the difficulty by counting only on the exhalations. If this becomes easy, drop the counting altogether and just let the attention rest with the breath.
Try to do this for a minimum of twenty minutes a day.