Which condition is necessary for a mild fossil to form

First, the organism needs to be buried quickly so the bones do not rot, sinking in tar, sand storm sinking in mud etc. Over time more sediment will cover the organism, preventing decay.

The mold fossil must be filled with sentiment. After filling mold fossil with sediment such as tar, heat and pressure causes tissues of organisms to release oxygen and hydrogen, as a result a residue of carbon left which create a dead impression of the dead organisms. this whole process is known as carbonization.