Let's look at a hypothetic fossil locality to see this process in action.

100 million years ago

Scallops and other invertebrates inhabit the sea in this area.

When they die, they are covered by sand that settles on them.

75 million years ago

Pressure compacts the loose sandy sediments into hard sandstone.

Dinosaurs now live, and die, in the locality. Like the scallops before them, their bodies are covered by sand..