For detailed instructions on the proforma invoice and how to issue an invoice from it, see the article prepared by the invoice module provider: InvoiceOcean.
Proforma Invoice - what is it and how to issue a VAT invoice from it?
1. Open the side menu, Invoices tab [1] -> Open [2].
2. Select Income [3] and then Proforma Invoice [4].
3. From the list, select the proforma invoice based on which you want to issue a VAT invoice.
4. Then click Copy to VAT [5].
5. If there is a need, you can now make changes to the invoice.
6. When you have completed the verification/changes, click Save at the bottom of the page.