QBO is accrual accounting. NOt much else you need to do other than if you wish to use billing and invoicing (vendor and customer management may require Essentials or higher)
Unless you close your books as of 12/31/19you can enter dates for any transaction going back as far as the software allows (doubt you can enter the original Columbus Day (10/12/1492) but I could be surprised.
By entering a sale without payment it is in effect accrued revenue as it is income claimed today under accrual rules. Invoices accomplish that.
