From the Class screen, simply click or tap on a new, empty Deck, and it should bring you to the flashcards editor screen. (If it doesn't, then you can access the "Edit Flashcards" option from the Deck's drop-down menu.)
The default editor is what we call the Simple Editor. This screen allows you to easily create short, text-based flashcards (e.g. for vocabulary) and quickly fly through the creation process. To exit the editor, simply click Done or Save, and your flashcards should be ready to study.
If you are on the website and you would like to add or upload images to flashcards, or change text formatting, then you will have to switch to the Advanced Editor, which is a feature of Brainscape Pro.
Note that your edits will be permanently saved once you click or tap the Save (or Done) button, and all updated content will be kept in sync between the web and your mobile devices.