Start by walking the site together. Point out every entrance, restricted area, patrol route, alarm point and place where visitors wait. A physical walk-through prevents assumptions and gives the guard a clear picture of the post.
Explain who may enter, at what times, and what identification or approval is required. Show how visitors, deliveries and contractors are recorded. If different rules apply after normal working hours, write those differences down clearly.
Give the guard emergency contacts and an escalation sequence. The brief should explain whom to call first for a medical issue, fire, access dispute or suspicious activity. It should also state what the guard must record while the facts are fresh.
Set out the shift handover. The outgoing and incoming guards should review keys, equipment, open incidents, expected visitors and anything unusual at the site. Put the full brief into a written post order kept at the post. Review it after the first week and update anything that proved unclear or impractical.
For support with this subject, see security manpower or contact Spectre Watch.