I decided to put this in the "Gore" section, cuz usually when someone come out of a grave they're all decayed and stuff and its usually pretty gross. Cool, huh?
First, dig a hole, then shore up the hole by building a wood box to line it (so it doesn't fall in) then either cover the hole with spandex and sod or fill the hole with soft moss and peat moss from a plant nursery... that's all there is to it.
If you use spandex: stake the edges tight and cut a slit big enough for the talent, then cover with sod and moss.
Also, if you like, get an old vaccuum hose,plug one end and drill holes in it. (1/4" is O.K.) Then pipe a smoke machine into it to have smoke as the corpse emerges from the hole.