The method is not intuitive and the lack of any meaningful help ensures that it is even harder. To add items, go to the Library listing and if you see the "Edit" button at the bottom, tap it. You will then be prompted to select items in the list. After selecting one or more, an "Add to…" button will appear. Tapping that will allow you to add the selected item(s) to a collection. Given that "Edit" is not an intuitive label for adding things to a collection, I was just lucky I stumbled onto the answer.