Go to Income > Sales Orders, find the open order and click "Issue receipt" in its action menu.

You will be directed to the receipt editor screen, where you can see the data in the original order are automatically brought to the new receipt and the reference number field is filled for quicker reference. 

There will be additional fields for you to fill before making this receipt complete, including payment information, currency rate, tax invoices, etc.

Of course, you can add some changes to the receipt if the final sales differs slightly.

On more details to create receipts, read this article.