Navigate to Team > My Team > Employee Information

Select the desired employee.

Navigate to the Payroll  tab > Payroll Alerts.

This will jump you down the page to the Payroll Alerts box. Select Add in the top right.

From this window you can choose which payroll you want the notification to show up in, either the next one or all of them going forward.

If you choose "Require Sign-Off" then you will not be able to close the payroll until you have marked this item as complete

After you save it, you can go back in to edit it or delete it from the same box