Overview
Trailhead has a nice overview of Workflows https://developer.salesforce.com/trailhead/force_com_declarative_beginner/business_process_automation/workflow including this video
Tips
Triggering Workflows to run on existing records
https://success.salesforce.com/answers?id=90630000000gmtfAAA
- Create the workflow to run to "subsequently meet criteria"
- Create a checkbox field called TriggerWorfklow - set it to false.
- Ensure the Workflow has TriggerWorkflow__c = True as one of the criteria.
- For each of the records that needs to have the workflow run on set TriggerWorkflow__c to be True.
- You can do this by importing a list of the ID's and a value of True.
- Now, when all the records are happily sitting in the Time-Based workflow queue then you can modify the workflow to remove the TriggerWorkflow from the criteria and even delete the field.
- Or set TriggerWorkflow to be true on all existing records.