Navigate to the Purchase tab and select Debit Notes to go to the Debit Notes Summary page. From there you can select New Debit Note and populate the required fields (Vendor, Currency, Date) then populate any of the desired optional fields (Discount Type, Reference #Admin Note), and then add your line items by typing in the item name, description, quantity, rate, select if the item has tax, and then select the checkmark to add the line item.
As you add items you will see the subtotal and total. In between these you can add in a discount amount as a percentage or a fixed amount (make sure you selected the discount type above) and enter in any adjustment amounts as required, leave any vendor notes and terms and conditions, then Save your Debit Note.
Once you select save you will be taken back to the PO Summary page where you can select a debit note to easily review the contents. When in this view you can edit, view as a pdf, send this via email, apply to an invoice, or select the more button to refund, void, attach a file, or delete the debit note. You can also review invoices debited, refund details, and set debit note reminders.