Anapanasati is essentially Samadhi focused on your breathing. In other words, your breathing becomes the object of meditation and concentration. Like Samadhi, this is a skill that requires practice. Also like Samadhi, Anapanasati has the benefits of relaxation, stress reduction, and building the capacity for maintained concentration. In many ways, Anapanasati is the final step before beginning the practice of Vipasanna.
To practice Anapanasati, begin by taking a few deep breaths to relax. Now, breathe normally through your nose and pay attention to your inhalations and exhalations. Try to find a spot where your breath is most noticeable. This may be your chest, belly, throat, the tips of your nostrils, or elsewhere. Follow your breath as it moves in and out of you. You may want to try counting your breaths (1-10 and then start over). You may also want to name your inhalations by saying to yourself, "In" and name your exhalations by saying to yourself, "Out." If you need to use counting or naming, use them as tools to help you maintain a focus on your breathing. Do not let them become the object of your focus. With practice, you may no longer need these tools.