OPTION 1 : from the settings
Click on "How do I customize my style guide?" for more details.
OPTION 2 : from an invoice
Go to the INVOICING section then select an invoice.
Click on GO TO INVOICE
The INVOICING SETTINGS allows you to update your invoice if necessary. SAVE to apply the changes.
To customize the template, click on STYLE GUIDE
Create the template of your files : font, primary/secondary header, footer and add your logo (1.)
You have customized the invoice template !