Sit any way that you feel comfy, so long as your spine is straight. Close your eyes and focus on your breathing. Take very long and very deep breaths through your nose.
Each time you breathe in, hold it for 2 or 3 seconds and then slowly exhale. Continue this breathing until you feel very calm and peaceful. This usually takes 3-5 minutes ( but that is just me).
Take as long as you need. It is different for everyone. Just go at your own pace. You can also use sound to center yourself. Just sit quietly and ring a small bell or chimes, or use a Tibetan singing bowl. Focus on the soft sound and breathe deeply. Keep making the noise until you feel centered.