Sing from your chest voice. Find this voice by speaking in an ordinary voice. Say your own name, then sing it using the same muscles. Talk as if you were speaking to a baby -- in a high-pitched, cutesy tone -- to find your head voice. Practice using this voice and changing between notes in it. Combine the two voices by starting with your chest voice and allowing the note to resonate in your head voice. This combination is referred to as your "mixed" voice. Practice singing scales in this note.
Sing lower and lower notes until you begin to lose power. Sing lower notes after they are produced by a piano to ensure you are hitting the right pitch. When you begin to lose power, generate power from your stomach by tensing the muscles as if you were about to get punched in the stomach. Send this energy up into your throat with the aid of your pelvic muscles. If you have done this correctly, your vocal will get more volume on lower notes this way.
Feel the resonance from low notes in your face. Practice feeling this resonance by humming. If you can't feel vibrations in your face, slowly shake your head from left to right. This should relax your throat muscles and allow you to feel the resonation of the notes. Look towards the floor while you hum if you can't feel the resonance. Do this each day for a week, or until you can feel the resonance. Being able to feel the resonance is vital to having control over the power generated by your lower register.
Sing "mee" in a comfortable pitch. Pick a note that is in the middle of your vocal range. Aim for a clean, non-vibrato tone and try to feel the resonation of this note in your face. Sing lower and lower notes. You will notice that your volume decreases as you go lower, despite the extra force you generate with your stomach.
Move to lower notes gradually, and focus on singing them cleanly and recognizing the resonance. Control the resonance of the notes to avoid losing pitch when you increase your singing power. If you can't get louder on very low notes, even with additional help from your stomach, hold the microphone closer to your mouth when you sing. Control the resonance of your notes and stay on pitch. Lower notes relax your vocal chords, so singing them very loudly is extremely difficult.