Select [Management] > [Document reminder management] > [+Create]. Choose the document class, choose [Expired Date] in the "Reminder time" column, and choose [Before] in the "Mode" column, and enter the number of days.

As for "Reminder subject" column, if you wish to check "Use the corresponding document class field," don't forget to create a column in the document class beforehand, and choose "Select member" in the UI Control column. At last, fill in the "Reminder email title prefix word" and "The reason for reminding" columns, and [Save] it once finished.