ok. this is partially mine, partially someone elses. whos name escapes me.
basically, you take peice of thick THICK paper, like for presentations. not cardboard. and you draw/trace a picture onto it. say a bat. you use an xacto knive to cut the bat out.
now take doublesided tape and go around the edges of the bat very very well. you dont want the spray paint to bleed. now put it onto your grave and spray it with black spray paint, white gray whatever. just get the 99 cent cans. the spray will eat away at the foam. spray it generously 2 or 3 times. the key is to do an overly good job getting the tape right against the edges. that way, the spray only eats away what you carved out of the paper.
any pumpkin stencil would work too. i did it with mine, its wicked amazing, but be warned. if you spray it like 5 times, it will eat all the way through to the other side. then let it dry for a few minutes and you can remove your paper and tada.
i did it to 6 of my 7 graves, its awesome.
get 1" foam.
edit: i just made this grave a few minutes ago from 1" foam.