You have identified the timing of the events associated with the clandestine grave. Now you must collect the evidence so you can analyze it.
Remember, evidence is useless if it doesn't reach the analyst intact! After collecting your evidence, you need to ensure it is preserved properly and transported safely to the forensic laboratories. For example, recently excavated bones can be moist. They should be stored separately in paper bags to allow them to dry and stay dry. A cardboard box should be used to protect the skull during transport.