Sing with a piano on a note that is in the middle of your range. Using whole notes, which last for four beats, extend your range by singing half step sequences. Use the following sequence to lower your pitch: if you start on C, the sequence is C - B - A sharp - B - C. Going down to A sharp and then up to C.
Continue to sing this progression at a slow pace, each time lowering your pitch a half step. If you start on C, then the next sequence will start on B, then A# and on to A. Continue this series until you find the lowest part of your range. Don't strain to sing lower than you are able to.
Take a five minute break. Start the exercise again a fifth below the lowest note from your first session. If your lowest note was a D, you will start on an A. Complete the exercise again, this time attempting to go one half step lower if it is comfortable to do so.
Practice this exercise daily to improve your low range. It will take time to expand your voice. Be patient and don't rush this process.