How do I make it look like I'm cutting someone's arm in a film?

I am a college film student and in a short film I'm making, there is a shot of a man slowly cutting a girl's arm (it's a pretty weird, messed up film... don't ask :P) but I don't know how I would go about doing this. I would like to be able to pull this off during the actual production, NOT post production (editing, CGI, etc.). I will be using a prop knife. What materials will I need, and how can I do this?

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