How are grave stones placed on a dead persons grave after the family has left the cemetery & their grave has?

been filled with dirt. I was looking at a photo of my grand mothers grave and I got to wondering is her casket standing up right in the ground or what because I got to thinking of you remove the grave stone(has her name on it and so and so) you could see her casket standing up up right. Do they place the grave stone in the middle of,UGH so confusing.