Basically, they'd sew you into a leather bag with various live animals, then throw you into the nearest and deepest body of water. Snakes were an early favorite, but documents also mention dogs, monkeys, and roosters. You'd be plunged into a struggle where, regardless of whether you or Foghorn Leghorn won the sack fight, everyone still drowned.
And no, the Romans didn't pick these animals for their comedic value. They believed these crimes "polluted" the community, and animal sacrifices were a way of cleansing it. It was more about appeasing the gods than deterring any monkey-phobic parricidal maniacs lurking about town. These sacrifices would also deny the condemned the "light of heaven and earth," with the snake even adding an infernal symbol. Which honestly makes the punishment that much more metal. Not only did the Romans force you to die a watery death while a monkey ripped your face off, but they also straight up thought they were damning you for all eternity. After a few centuries, they did eventually say, "Hey, maybe this is kind of a bit much," and call it off.