An invoice is a bill that you can give to your clients, asking them to pay for the goods or services that you sold to them. As a writer, you would invoice a magazine for an article that you write; as a marketing consultant, you would invoice a business that you contract for etc.
You can create it as a Word doc, as a PDF or any other document type. Make sure that it’s not too niche (e.g. a screenshot or image file) as the finance team of the business you’re invoicing may not be able to accept it.
If you’re self-employed, make sure that it includes the following information:
You don’t only use invoicing for payment but as a general method of tracking.
If you’ve registered for VAT, the invoice needs to include more detail – read more about VAT invoices on HMRC’s website. You can issue an invoice either before or after you’ve supplied your customer with a good or service.
There are plenty of free invoicing templates and tools online which will make it easier for you to create something if you’ve not done it before.