Begin by lying on your left side in a comfortable position. It's important to make sure your body is in a position that allows your muscles to feel relaxed, so use pillows if needed to make sure your head, neck and spine are comfortable.
First focus on taking a deep, slow breath, allowing your belly to expand. Exhale slowly, feeling the airflow out of your mouth. You can use words as your breath if it helps you to focus – try saying "he" as you inhale and "who" as you exhale.
Visualize stress flowing out with your breath or any other thought that makes you feel calm. Some people like to think of their breathing being like the tide, slowly rolling in and out.
Once you have mastered "belly breathing" you can use your breathing to relax your entire body. While lying in a comfortable position start with some deep breaths and then focus on relaxing the muscles of your neck and shoulders. Visualize your muscles relaxing and your body sinking down into the bed. Move slowly down your body, relaxing arms, torso, hips and legs while breathing slowly and deeply.
Don't force your breathing and if you lose your concentration, relax and try again later. You should practice "belly breathing" for 10 or more minutes, several times each day. You'll find that with practice you'll be breathing more smoothly and naturally throughout the day. Remember to use your breathing during stressful moments to help reduce anxiety and muscular tightness.