Please refer my previous article to learn more about SharePoint Designer 2013 Workflows.
In this blog you will see how to use “Extract Substring from Index of String" action in SharePoint Designer 2013.
Description:
Copies a substring starting at a specified index in the string and places the value in a variable.
Note: The index value in Microsoft SharePoint Designer 2013 is zero-based.
In this example, I have copied a substring starting at a specified index 3 from “Name” field and storing it in a “ExtractSubstring” string variable and updating the current item “Notes” field to display the output string.
Workflow Design:
Result: