She makes it a habit to drink a glass of milk every morning.
Literal
She [topic-は] every-morning one-cup [possessive-の] milk [object-を] drink [habit-ことを常としている].
~ことを常としている is a formal/literary way to express 'makes it a constant habit.' 常 (つね, 'constant, usual') elevates the register. 一杯の牛乳 uses the counter 杯 (はい) for glasses/cups.