The template has a client database built into it so the first step is to add your client list via the "Client Management" section at the bottom of the notion template.
Once you have your client list in, you can then add the invoice data for each client.
Then, all you have to do is fill in the: Invoice Date, Due Date, Invoice Amount, and when an invoice is paid: the payment date.
If an invoice hasn't been paid
If your client hasn't paid their bill yet, and the invoice is overdue, then the table will show how many days the invoice is overdue by and how much money is currently outstanding.
When an invoice is paid
Once an invoice is paid and you add the payment date to a record, then the table updates to show you how many days the client took to pay the invoice, along with how many days overdue it was.