Good examples and a good explanation of Simplerinvoicing UBL Invoices are published on:
Wiki UBL Invoice (factuur)

These examples are created during the national Ketentest and gives examples of how different software vendors are creating invoice and what they expect to receive as invoice.

There are also examples for how to use specific elements in Simpler Invoicing Standards.

With the validatethis service form Simplerinvoicing  you can check your documents