How to create/assign category to Flashcard Slider?

You can either create a new Flashcard Category while you Edit Flashcard / Add New Flashcard. On the right side => there is a ‘Flashcard Categories’ metabox which has ‘Add New Category’ link at the bottom.

Also, you can create a new Flashcard Category thru the Flashcards admin menu => Flashcard Categories (on the left hand side of the WordPres admin below ‘Commnets’ admin menu).