How do i do this tommorow?please reply ?

My cousin is so yummy!she has a nice booty I always stare at it and I always have wanted her to facesit me with her booty and tommorow my phone has a cam recorder and I wanted to film her booty when she's bent over so I have fun to the video when I get home so how do I film her booty without her noticing am using a phone cam btw so how do I do that?the reason when I wanna film her booty when its bent over cus it looks tight when she's bent over and don't say its your cousin or incest or something like that how do I film her sexy butt without her knowing