Hi there, @shaun7!
You can import your custom invoice template directly from the Custom Form Styles page. I'm here to help you accomplish this.
First off, you'll have to turn the Import Style feature from the QuickBooks Labs page so you can import your custom invoice template. Let me show you how:
Once completed, here's how you can import your custom invoice template:
In addition, here's an article you can read to learn more about how you can import your custom invoice template: Import Custom Form Styles for Invoices or Estimates.
Lastly, I've also included this helpful article in case you need to add and customise a logo for your custom invoice template: Add, Customise, or Remove Logos on Sales Forms.
If there's anything else that I can help you with, please let me know in the comment section down below. I'll be always around ready to help.