I went into a bit of detail for the Rancher NFS on-top-of AWS EFS approach in a different thread: Convoy EFS to Rancher EFS
As for the normal Rancher EFS on-top-of AWS EFS, well that’s pretty straightforward. You just go to Catalog->Library in the Rancher 1.2 UI and select the Rancher EFS service. Then enter your AWS secret and access keys. Then every persistent storage volume you create at Infrastructure->Storage will create a new EFS file system in AWS. The Rancher docs show how to use those storage volumes in your stacks and services.
As mentioned above, when using the Rancher EFS -> AWS EFS approach, AWS does start you out with a limit of 10 EFS file systems per account, so you may hit that pretty quickly depending on your usage scenario.