Take a big breath of air and pull air into your stomach. This will ensure that there is enough airflow to keep your vocal folds vibrating.
Sing a pitch near the bottom of your range. Go up a few pitches higher and hold the note out for four seconds.
Descend to the lowest note in your range by sliding through and blurring all of the notes in between on the sound "hmmm." Don't worry about singing actual pitches, the goal is not to become a singer but to gently stretch the vocal folds to lower your vocal range.
Return to the original pitch you started on and attempt to go lower. Don't strain your voice, this is a delicate process. Simply going near the lowest note in your range will expand the vocal folds over time.