Because a little girl's urinary system is more exposed, making the area more prone to irritation and infection than a little boy's, diapering her is different, too.
Cleaning
Clean your baby's diaper area from front to back, and pat dry.
Amount of Ointment
Place diaper rash ointment in your hand, keeping in mind that applying too much could cause leakage. It should be room temperature or above.
The Vaginal Area
Spread ointment on your baby girl's vaginal area anywhere the skin is visible to your eye--the flat outer area, and labia majora (outer lips). Never insert the ointment into the baby's vagina, or anywhere the skin is naturally a darker red.
The Bottom
Spread the ointment sparingly over your baby girl's bottom, as you would a boy's.
Creases
You may also spread ointment along the creases (or folds) of the baby's skin.