Every time that Sato went to the Masonic rituals, it was in disguise; he put make-up on to cover his massive tattoos, and put a wig on to cover his lack of hair. His intents were to undermine the Masons, to reveal their secretive rituals to the world, and to use the knowledge that he gained through participation for his own gain and power, which he wanted in order to enact destruction.
In the book, it is when he goes to his home for the first time that Dan Brown reveals how he got video of the rituals. As Sato is putting on his disguise again, before he meets Katherin the first time posing as a doctor, he puts his wig on. As he does so, he mentions that he had devised a video camera that he hid inside of the wig; that way, everything that he saw in the rituals, the video camera that was placed in his wig also caught it. So, it is through his disguise, his wig, that he captures video of the elusive and enigmatic Masonic rituals.
I hope that helped; good luck!