Go to the Sales -> Documents and click on the "Create a New Document" button.

Select "Proforma Invoice" as the document type. 

Data entry is similar to creating a new sales invoice.

From proforma invoice it is possible to create a new sales invoice. These options are located at the bottom of the form.

"Save as Draft" option will create a new invoice. You can offset such invoice later by clicking on the "Add a Payment" button and going to the "Use Prepayment" tab. 

"Post/Use Prepayment" option will create, post and offset the invoice in one click.