These are the steps on how to customize the invoice template to show the fields:
For custom templates, editing them can be done in the original form (MS Word or the DOCX file). From there, edit the fields and map them correctly. Then, re-import it to QuickBooks Online.
Please refer to this article for the steps: https://community.intuit.com/articles/1764577-import-custom-form-styles-for-invoices-or-estimates
Come by once more if you've got more importing questions.