Go somewhere quiet and lie down or sit in a chair with good back support so that your body is straight, allowing you to breathe deeply and comfortably. Make sure you are not wearing any tight clothing. Tell yourself to relax. Work on every part of your body from the top of your head to your toes, consciously relaxing each part. Concentrate on your breathing, taking deep in-breaths, through the nose, letting the breath fill your body, relaxing and calming. Then breathe out through your nose or mouth, exhaling all negativity and stress and ridding your body of the toxins which build up there. Repeat this a number of times - if you can only manage 10 minutes that is fine. Eventually you will be able to practice this anywhere, any time, when you are feeling stressed. Try not to hunch your shoulders as you breathe in - try to consciously relax the shoulders and neck and breathe from the abdomen - watch your tummy expand as you breathe in.
There are a number of breathing techniques which are taught by practitioners of different disciplines, either on a one to one basis or in a group, such as a workshop perhaps. If you have a particular medical problem, then a good place to start is with your doctor, who will be able to advise you.