Load the Cashier Maintenance program:
Now, click into your cashier of choice, and set the MCR Operator Group of your choice. The options are Cashier, Manager, Supervisor, Matchday Staff and Non-Till Operators:

Each of these have a pre-set permissions:
Now looking back at an individual in Cashier Maintenance, when making any changes here, or creating a new cashier, you must only click the Update Cashier Details Only button:

Note: The Permissions Options at the bottom of the Cashier Maintenance program have no bearing on permissions, as these only apply to the old software: