In the bookmarks menu (the open book icon), press the + button at the top to open the bookmark creator, and type in a title as well as start and end pages. If something is wrong with the values you’ve entered, the bookmark validator will alert you. Once any errors have been corrected, tap the Save button to finalize your bookmark.
Tip: to create a traditional bookmark that simply opens your file to a particular page, leave the “to” page field empty. Note that these bookmarks are not listed in the score menu.