A920 & A80’s: 

In the Payment app you can go into the Settings, then Application Options. Once there, scroll all the way down and enable Clerk/Operators. That will make it mandatory to enter a clerk code before a processing a transaction. You'll then have to configure the codes, names and passwords.
 

In summary, Payment>Settings>Application Options>Enable Clerk/Operators. 

 

Legacy Terminals:  

For Legacy terminals the merchant has to exit ECR mode by pressing the Yellow button and entering their password. Next they have to enter the Admin menu by pressing the Up arrow, in most cases, or the dash button top right.
 

Once there, to activate tips: 

Press #3>Press #1 and press the Green button until reaching the Tips option (Basically, anything that doesn't need a modification can be skipped by pressing the Green button). 

  

To activate Clerks:  

Press #3>Press #5> Follow the steps.