1.  Go to *Sales \> Invoicing \> Orders to Invoice*.
2.  Select one sales order to invoice. If none, use usual sales flows to
    get one.
3.  Click on *Action \> Invoice Order*, or access it and click on the
    "Create Invoice" button.
4.  On the popup that appears, select "Percentage of the quantity" on
    the "Create invoice" selection, and put a percentage quantity.
5.  Click on "Create and View Invoice" button.
6.  On the newly created invoice, you'll see that the invoiced quantity
    is the expressed percentage.
