How do I add a line above the title and a letter to the couple?

A few words of your own on the cover and under it, and a line or two for each chapter.

Open Design › Words.
Line above the title: the small line over the couple's names. Left empty, it follows the gallery's type, like “The wedding of”, or shows your studio's name.
A letter to the couple: a few warm words shown with the cover. Leave an empty line between paragraphs.
For a chapter, open the section's ⋯ menu on the Photos tab and choose Text under the title….