When I first heard this story, I had thought that hopefully he had removed the scrotum first, and that perhaps there was some ambiguity as to whether or not it was actually the artist's scrotum, but no.
At least the article still leaves some mystery for the precise mechanics. I mean, what kind of nail would you need to go straight through cobblestones? What kind of hammer? Surely one couldn't literally nail something with one's bare hands? I should stop thinking about this.