In the context of the four ordinary ngondro practices I have done the impermanence meditation of nine cemetery contemplations. It is described in the Satipatthana Sutta and in the Vishuddhi Magga. These days you don't see bloated or blue corpses, or sceletons etc.. lying around, in cemetaries or elsewhere, in Europe. So you have to do this meditation as an imaginary exercice, even so it is very effective. It is important to do it in the context of the other three ngondro practices. And sometimes you have to combine it with more normal meditation topics, like the Four Brahma Viharas, or just developing mental calmness.
Besides the descriptions that are found in the Nine Cemetery Contemplations, there are usefull descriptions of corpses and cadavers in the wikipedia, these can be used when doing this meditation: