There are different ways to perform focused breathing. Among the differences are sitting positions, in Yoga there is an Easy Pose, Lotus Pose and Corpse Pose, in Karate and Aikido there is “Seiza” position. In any of those positions you slowly, very slowly, breath in and out. In yoga, in sitting position, you place your hands on your knees and let your thumb and index finger in both hands to touch forming circles. The "circle" helps you picture a flow of energy and that also helps you concentrate on breathing.
Be a normal person and find the most comfortable position for yourself. Just remember to straighten your back, don't slouch, because sitting straight will allow you to feel your whole body, your physical presence.
Breathing. Since it is a focused breathing you have to focus on it. When you inhale you start by expanding your stomach and then you move up to the lungs and then top of the lungs. When you can't do it no more you stop for 5-10 seconds and hold. Then you slowly exhale. You can make some noise while exhaling. The exhale is longer than the inhale. Then, when you don't have any air to push out hold for 5-10 seconds. Now, repeat.