This is the standard for handling a crime scene when first alerted and arriving or happening upon a crime scene:
Call for backup if needed.
Remove non-essential personnel and civilians and set up a perimeter using a Do Not Cross Line, barricades and the like.
Participate in a Crime Scene Briefing with attending officers, setting up expectations and the details of investigation, so that they may aid you with collecting clues and witness accounts.
Take brief statements from witnesses to ascertain their time of arrival, relation to the scene and the events, and any helpful insights and discrepancies.
Document the crime scene discovery with witnesses (Who, where, when, with whom, and how.)
Document anything and everything in the scene prior to it being disturbed.
NOTE: If the officer arrives before emergency teams (medics or fire-fighters) arrive, this should be done right after removal of non-essentials, since once the emergency teams arrive, they will disturb every bit they need to get their job of saving lives done.
Document circumstantial data, and impressions of the scene in notes like personal effects found at the scene, location and position and conditions of the body\bodies, identifying marks of the victims. Document all data collection methods used, from physical manipulation, Esper Scans, ocular scans, face scans, DNA and fingerprint sampling.
Look for signs of scene tempering, evidence of movement of furniture, any other objects, and the bodies themselves.
Document injuries or lack thereof, the condition of the bodies, rigor, temperature, decomposition. Identify what other details are needed or expected for the coroner's examination. And identify the similar to determine what physical evidence is needed to establish the cause, manner, and circumstances of death.
Based on findings establish need for further forensic collection, examination, or analysis.
If crime scene has relation to other departments, or agencies of the UN, inform officers of that.
After walk-through of the crime scene, You can begin final documentation, photographing, securing, and preserving any evidence and trace evidence (hair, blood, fibers, etc.) with proper containers labels and preservatives. Make sure all property and evidence is collected and safeguarded as required by the law. You may require for Crime Lab assistance with all of this.
NOTE: Not all of these steps are relevant to all officers. All officers are however required to make sure that steps 1 to 6 are being performed. Very often, a Crime Scene Officer will be assigned to the scene, that maybe the detective that heads the case, or anybody else. They will have the authority and responsibility to see that all of the 12 steps are accomplished.