Start with the risk assessment
Think about the work, the place and the person: working at height, confined spaces, live electrics, violence from the public, medical conditions, poor signal. Some work shouldn't be done alone at all.
Set the interval
Higher-risk work needs more frequent check-ins. Agree the interval before the job starts, plus a final check-in when the person has finished and left.
Plan for a missed check-in
Write down the steps: try to call, try again, contact someone nearby or the client, go to the site, and when to call the emergency services. Everyone needs to know the plan, and who to escalate to out of hours.
Keep a log
Record each check-in and every escalation. It shows the system works, and it helps spot jobs where check-ins are routinely late.
Know the limits
A check-in log depends on someone watching it. For higher-risk work, consider a lone-worker device or app that can raise an alarm automatically.