How to implement shallow cutting mechanic?

First of, TW for discussion of self-harm. Not sure if this is a noob question or not, but I'm developing a (3d) game that includes a self-harm mechanic. I would like to have the player drag the blade over the flesh and product shallow cuts (something similar to this website, except on a part of the character model in 3d). I'm not really sure how to go about implementing this, so any help would be appreciated!