I fired my employee after they had only done one day of work this week, do I mark them as a leaver and then do the final payroll, due holiday or do that first?

Hi there, @Babezipllyn.

Before marking an employee as a leaver, you'll need to handle their final payroll first. 

I recommend processing their final payroll first to capture their accurate earnings and deductions, then marking them as a leaver in QuickBooks. Once you reach the employee’s final payroll, you’ll need to send the FPS (Full Payment Submission) to let HMRC know that you’ve paid the employee. If you’ve already paid the employee, the next FPS submission will include all the final details.

Feel free to read this article for more information about issuing a P45 form: Issue a P45 form in QuickBooks Payroll.

Please don't hesitate to leave a reply if you have additional concerns with QBO Payroll. We're always ready to help.