From Meditation, insight, and rationality (Part 2 of 3):

Basic method: Sit down in a place where there are few distractions, and pick an object to focus one's attention on. The most popular objects are the feeling of breath at the tip of the nostrils / upper lip, and the motion of the abdomen as one breathes in and out. (In this description I'll assume you're using the latter.) Begin by trying to clearly perceive the feeling of the abdomen expanding and contracting; when it expands and you perceive it clearly, attach the label 'in' to that perception, and when it contracts and you perceive that clearly, attach the label 'out' to that perception. As your attention becomes more stable and precise, you can divide the experience up into as many parts as you can discern: for example, 'in'->'holding'->'out'->'holding', or further, 'in-beginning'->'in-slowing'->'holding'->'out-beginning'->'out-slowing'->'holding'. The label you use is not important so long as it's simple and makes sense to you. What is important is attending to the perception, and the best way to do this is by attaching a label to the perception every time you notice it clearly. Focus on perceiving every aspect of the movement of your abdomen as precisely as is possible for you, given your current level of attentional and perceptual development, and on keeping your attention as set on the movements of your abdomen as possible given the same. When you get good at this, try to incline your mind towards the attentional / perceptual flux called 'vibrations' in the experience of your abdomen moving. Try to see how, in the experience of attention being fixed on an object, it is continually being set and re-set there. After enough practice, they will make themselves apparent.

Whenever your attention goes to anything other than your abdomen, attach a label to the accidental object of attention and then go back to your abdomen. If you wonder about how effective the exercise is, believe it's easy, believe it's hard, decide it's pointless, congratulate yourself for how you're doing, etc. etc., label it 'thinking' and go back. If you think about your day, your future plans or responsibilities, etc., etc., also label it 'thinking.' If you visualize what you're going to do after meditation, etc. label it 'imagining.' If you have the desire to move, label it 'restless.' If you form the intention to move, label it 'intending.' If you feel some emotion, label it appropriately: 'happy,' 'sad,' 'enthusiastic,' whatever. If your attention wanders off for a long time, when you regain it and realize that, label the whole daydream or reverie 'wandering' and put your attention back on your abdomen. If you hear a distracting noise, label it 'hearing.' Anytime your attention is not on your abdomen, whatever it's on, recognize that it's not on your abdomen and is instead on that thing, label it, then go back.