How do i customise an invoice? theres no 'tab' or link anywhere on my screen

We're glad to have you here in the Community, Heatheratp. I'll provide the steps to customise your invoices and incorporate your company's information.

Customizing your sales form helps present a professional image to your clients and reinforces your brand identity. Let's work together on locating the Customise tab. I've added a screenshot below for visual reference.

1. Select Customise.
2. To customise the form you're currently working on, select Edit current. For a new template, select New style. Or, to customise an existing template, select the template name.
3. Give your form template a name.

However, if you're already on the invoice tab but don't find this option. We can use another way of customizing invoices. I'll show you how.

1. Go to the Gear icon.
2. Select Custom form styles.
3. Hit the New style drop-down and choose Invoice. 
4. Click the Design, Content, and Emails tab to customise.
5. Once done, press Done to save customization.

Once you have a template set up according to your preferences, you can use it for all future invoices with minimal adjustments. Refer to this article for more detailed steps and tips: Customise invoices, estimates, and sales receipts in QuickBooks Online.

When you're ready to receive a customer's invoice payment, you may want to read this resource that will help you how to handle those: Record invoice payments in QuickBooks Online.

Keep us posted for updates or if you require additional assistance in customizing your invoices in QuickBooks Online. Have a wonderful day.