Get the album "Hannah Montana 2: Meet Miley Cyrus" and listen extensively to Disc 2. These songs are performed by Cyrus as herself, not as Hannah Montana.
Practice singing Miley Cyrus songs. Choose songs that might be easier to mimic, that don't have a lot of range or pitch. Try "East Northumberland High," "Right Here," "As I Am" or "I Miss You."
Use a slight country twang. Miley Cyrus is from Tennessee and her Southern drawl is evident when she sings as herself. Use a husky, throaty voice. Be strong and confident when you sing.
Sing rock songs. Unlike her alter ego, Hannah Montana, who sings more pop songs, the style of Miley Cyrus's songs lean toward rock.
Keep the range of your voice between a deep and a normal pitch. Work your voice deeper and deeper as you practice, starting with the normal pitch and eventually getting a pitch as deep as Miley Cyrus.