Before you can issue an invoice or any other document (and even after, if you have not already sent it to the customer), you can change how it looks on the page (the design or the template).
You may prefer that the invoice appear three times on an A4 or you also have the receipt on the same page with the invoice, or you need a design that allows you to invoice a long list of services. All of this and a few more are found on the invoicing page, just below the invoice fields.
Each type of document (invoice, proforma, receipt, notice) has its set of templates, and for each one you choose following the same steps.
To view the document, you need to save it first (first, we suggest you save it as a draft).
To automate the process, you can also set a default template, that will apply automatically to each document you issue.