From my understanding there was a moment where he talked about his art of puppetry. First he killed the shinobi, drain the blood and take all the organs and veins. Let it sit in the cold for a certain while which is the hardest process cause the skin might break and start deteriorating. And lastly good ol poisen and hidden traps. And you got yourself a human puppet. Sasori was a strange guy. But yet he had passion of eternal beauty. He took time too make over 100 puppets and even himself. Which he used some sort of jutsu to store his soul into it. And veins throughout out the puppet. But all in all he was a pretty cook Akatsuki member.