Hello there, Thecobshopltd.
Are you using QuickBooks Online Payroll subscription? Or PaySuite?
I'll provide you with the articles on how to mark your employee as a leaver:
If you're subscribed to QBOP, here's how to issue a P45 form:
If you're using PaySuite, here's how to issue the form:
If you have other concerns, just leave a comment below.