1. This option will only work for setting automation on Lead, Account, Contact or Opportunity. If you wish to setup automation on custom object, you will have to use Process Builder or Apex method.
  2. There are two fields on Account, Contact, Opportunity and Lead that come with RS Documents package: RS Document to Merge & RS Document Error.
    1. If you can’t see the fields, edit your layout and add the fields to layout.
  3. Trigger Document Action:
    1. If value of RS Document to Merge field is changed, then Document Action will be executed. This can be easily setup with workflow rule based on any criteria you want.
    2. RS Document to Merge field should contain Document Action ID autonumber (e.g. GDT-000002) or Salesforce ID of the Document Action (either one will work).
    3. If there’s no errors, then Document Action will get executed and RS Document to Merge field will get emptied out.  
    4. If error happens, error message will be saved in the RS Document Error field. Try to fix the issue and set criteria to trigger execution again.