First, you tear off its head. The Triceratops was a herbivorous dinosaur that was about 30 feet long, 10 feet high that weighed 6- to 12 tons. Its head sported a long bony shield which served to protect its neck. It inhabited marshes and forests of western North American during the Cretaceous period.
Triceratops was preyed upon by Tyrannosaurus rex,(T-rex) a carnivorous dinosaur measuring up to 40 feet long, 13 feet tall at the hips, and weighing up to 8 tons.
So how did T-rex get past Triceratops’ bony neck shield? Paleontologist Denver Fowler and colleagues, working in the Hell Creek Formation of Montana studied fossils of 18 individual Triceratops that showed tooth marks on the neck shield. “Specimens exhibit a suite of puncture, score, gouge, and puncture-pull marks, which in combination with tooth-spacing patterns are similar to traces previously attributed to tyrannosaurid theropods.” Fowler et al. surmise that T-rex latched on to the neck shield of the Triceratops and tore its head off to get to the meaty neck muscles.
An article in Nature by Mark Kaplan presents a series of graphics showing how this may have occurred.
Fowler and colleges also found tooth marks on Triceratops’ occipital condyle, the ball-and-socket head to neck joint. The tooth marks could occur there only if the Triceratops had been decapitated.
Fowler, D.W., Scannella, J.B., Goodwin, M.G., & Horner, J.R. (2012) How to eat a Triceratops: large sample of toothmarks provides new insight into the feeding behavior of Tyrannosaurus. Journal of Vertebrate Paleontology 32(5, abstracts vol): 96