See: this link for additional instructions. Notes tested here are below.
This is for Mac OS X (Snow Leopard) Installation.
Files to customize are located in
Changes to stylesheets will only be visible in the invoice upon restarting Gnucash.
Once you have created an Invoice (Reports → Business → Easy Invoice or Fancy Invoice) then you can apply a stylesheet using the Options dialog window.
You will have 4 options for stylesheet defined by corresponding stylesheet files (not sure the convention, why, or how yet):
To customize the content of the invoices, customize the following files (corresponding report name shown in Reports → Business is shown below). The name of the invoice as shown in the Report→ Business menu is also shown within these files toward the end of the file within the section “Define Report (define-report)”:
To add a new invoice, you could copy an existing invoice template (one of the four above) and then make sure to:
business-reports.scmfile (also located in the same folder).