Personas for Jira works out of the box without special configuration.
However, if you want to make the most out of this app, you can set up the Persona custom field to change assignments on the fly.
Notice that this only works for company managed projects.
Go to the Jira administration -> Issues -> Screens and select the appropriate screen for your project's issues.
Add the field "Personas" to the screen. You should now see this field on the right hand side of the issue view.