Projects is a fun and exciting tool that will make working with QuickBooks Online even easier! If you have multiple jobs on the go and you'd like to keep your income and expenses separated, Projects is a great option. I'd be happy to explain how to receive a payment for an invoice after creating a new Project.
If you created the invoice prior to creating your project, I'd recommend first linking the invoice to the project. That way, when you record the payment towards your invoice, the revenue will be linked to that specific job.
To edit your existing invoice, click the Sales tab, click Invoices at the top of the page, select the invoice from the list, then change the Customer to the Project.
Now that the invoice is linked to the project, you can follow the regular method to receive payment on the invoice. Once you've recorded your payment, this will be linked to your project.