Go to the Purchases -> Documents and click on the "Create a New Document" button.
Select "Proforma" as the document type.
Data entry is similar to creating a new purchase invoice.
From proforma invoice it is possible to create a new purchase invoice. This option is located at the bottom of the form.
You can offset such invoice later by clicking on the "Add a Payment" button and going to the "Use Prepayment" tab.