Summarize this article:

To produce a smooth, powerful, and natural voice, you must first learn proper breathing techniques. While humans are meant to breathe and speak from their diaphragms, many people do this with their chests instead, and this produces a weaker voice. You know you're a chest breather if you take a breath and your chest and shoulders rise. To correct this and to teach your body to breathe from the diaphragm:  Breathe in deeply for four seconds, making sure you breathe into your diaphragm. After four seconds, hold the air in your diaphragm for another four seconds before exhaling for a count of four seconds. Repeat this exercise for two minutes every day. Work your way up to practicing this technique for five minutes a day. Once you're comfortable with the four second breathing, you can practice the same technique, but extend the inhale, hold, and exhale to 20 seconds each. You only need to do this once daily. As you teach your body the proper breathing technique, you can start working on speaking from your diaphragm as well. To do this, take a deep breath into your diaphragm and practice speaking as you exhale and your belly flattens. As you begin to run out of air, take another deep breath and speak again, but be sure you only speak as your belly is flattening. When possible, always breathe through your nose. Air inhaled through the nose is moister and better for your vocal chords and vocal strength. Muscle tension in the body and throat or emotional stress can all wreak havoc on your voice, and this can lead to a thin and nasally voice that has no resonance (doesn’t carry well). The breathing exercises will hopefully calm your body and mind, and you can relax your throat by: Yawning deeply, and as you finish the yawn, begin humming. Open your jaw to a comfortable width and move your jaw from side to side as you hum. Do this for a few minutes, then use your fingers to massage your throat.

Summary:
Practice proper breathing. Incorporate proper breathing into your speaking. Relax your body and your voice.