Using possessive de when owner is not a proper name

I keep messing up on the possession concept. The lesson covers possession using proper names, but does not mention rule for non-proper noun, example, the girl's pen (unless I missed something). The correct way is apparently the pen of the girl, le stylo de la fille, and not le stylo de fille. Can you add a note to the lesson to accentuate this, such as found on Lawless site, https://www.lawlessfrench.com/grammar/possessive-de/