Create a Schedule as described in https://www.edureka.co/community/59616/snapshot-schedule-regularly-automatically-regional-persistent
Once you have a schedule, attach it to an existing disk. Use the console, gcloud command, or the Compute Engine API method.
Attach a snapshot schedule to an existing disk.
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Go to the Disks page in the Google Cloud Platform Console. 
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Select the disk you want to add the schedule to. 
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At the top of the page, click Edit. 
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Use the Snapshot schedule drop-down menu to add the schedule to the disk. Or create a new schedule. 
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If you created a new schedule, click Create. 
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Click Save to complete the task.