@kaz's answer sounds pretty much right to me; I just want to explicitly illustrate how her ability has worked in the cases we've seen so far.
Episode 6, first time
Magane writes in the book that the shopkeeper will be attacked by Hounds of Tindalos. This is a lie: Magane does not have the intrinsic ability to summon Cthulhuian monstrosities. Magane asks the shopkeeper whether he thinks that it's a lie; the shopkeeper replies that it obviously is a lie. Since the shopkeeper disbelieved her lie, her lie becomes truth, and so Hounds of Tindalos magically appear to devour the shopkeeper.
Episode 6, second time
Magane tells Aliceteria that her lance will end up piercing herself instead of Magane. This is a lie: Magane presumably does not have the ability to redirect weaponry. Aliceteria describes this statement as "nonsense". Since Aliceteria disbelieved her lie, her lie becomes truth. Had Selesia's arrival not prevented Aliceteria from making contact, she likely would've been magically run through by her own lance, given that Magane said her "a lie about a lie"line.
(It's unclear to me whether Magane actually needs to utter the "a lie about a lie" line for her ability to trigger or not. It might be just for show.)
Magane tells Mirokuji that his attacks are "nothing" to her. This is a lie: if Mirokuji were to land a hit on her, she would likely be in bad shape. Mirokuji calls her on her bluff ("I've never met anyone who was fine after getting hit with this"). Since Mirokuji disbelieved her lie, her lie becomes truth, and so Magane is now magically immune to Mirokuji's attacks, or at least his attacks with "this" (i.e. his sword).