Hiya Holly, hope you had a good weekend.
A self-billing invoice is when a customer prepares the invoices and then forwards them to you (i.e. the supplier) with the payment. In theory this should save you time in not having to issue an invoice yourself but it's of course best to ensure there's an agreement in place between you and the client that this is how you'd prefer to operate.
With this you simply record an invoice as you usually would, however, once you’ve issued it just remember that there’s no need to actually email or post it to them. This is just a process to make sure the accounting records are accurate.
If you're using Crunch software and need some help, check out our walkthrough on how to create a sales invoice.