Inserting a Sales Invoice manually

Good Evening,
i need to insert a sales invoice manually, that means not from a sales order or delivery note. The “Mapped Document” functionality requires another document as a base.
The basic idea was to fill the “Sales Invoice” and "Sales Invoice Item(s) Objects Attributes manually but:

  1. I think too many validations are passed that way
  2. I have no idea how to get the parent for the items before saving the invoice header what i can’t as the validations failed with missing “Items”

What would be the “correct” way within my scenario to do it.

Thanks in advance to everyone who can give me a hint.

Best Regards

Go to Selling > Setup > Selling Settings and set Sales Order and Delivery Note required as “No”