You can give the following
Post Process - AEM Worflow - Get Signers(This will depend on your workflow name)
Structure Payload - > Folder( This is the easiest option and recommended)
Data File Path - > Data.xml( this is the xml data of the CM letter that is stored relative to the payload of the workflow process
Attachments Path -> Attachments ( Attachments folder to store the attachments . This folder is relative to the payload folder of the the workflow process)
Layout Path - > can be blank
PDF Path - > Letter.pdf (The CM Letter that is stored. This node is relative to the payload folder of the the workflow process)
The path of the payload process is
etc/fd/dashboard/payload node.