How does Mary Shelley create a 'frame narrative' in her 1818 novel 'Frankenstein'?

In Shelley's 'Frankenstein', it should be initially established that the opening narrative is in epistolary form: that is to say that written letters form the first few chapters to give the sailor Robert Walton a voice. When Walton meets Victor Frankenstein, the latter then becomes the narrator and recalls his story to Walton, who acts as the reader by proxy. However, to further the complexity of the narrative, Victor recalls meeting The Creature on a mountain, and, for a brief selection of chapters, the creature assumes the role of the narrator in order to recite his story to Victor. We can imagine The Creature's narrative as the centre of the frame. From this, Victor returns to being the narrator- forming the middle layer of the frame- and, at the end of the novel, Walton's epistolary form is reinstated, forming the outer layer of the frame narrative.