Summarize this article:

To be professional, you start with your company's information at the top of the page. If you don't have a company name, just start with your name at the top. It can be centered or all the way over to the left. Treat the invoice like a business letter. That is, you can have a professional header at the top, centered in the middle. On the other hand, a simple statement of your business name is also effective. Put your address, phone number, email, and any other relevant contact information. Make sure to use your business information, not your personal information.  A fax number may also be relevant. If you accept payment services like PayPal, be sure to use the email you have set up with that account. Just like with a business letter, you now move on to the person you're billing. If it's a business, use the business address and contact information that the company gave you.  If it's a person, use what information you have, but preferably their business contact information. If you don't have enough information, you may need to contact the person or business with the information you do have to find out more. If it's a large company, you may want to call anyway to ask exactly who to address the invoice to. Create a unique identification number for each of your repeat customers. Then, make sure that this number is included on every invoice that you write for this customer. This will allow you to group invoices by customer and, if necessary, assess their payment history or document a consistent failure to pay. If you're sending out invoices on a regular basis, it's important that each one has a unique number so it's easy to reference and identify. How you choose a number is up to you, but remember that you must have a new one for each invoice.  One simple way is to just start out at one and work your way up. To keep the length of the number uniform, you could start with "0000001."  Another method is to assign a customer number and then use the date. For instance, if the customer number is 305 and the date is February 2, 2016, you could use 305-02022016 as your number. Just make sure you're clear on what the "date" should be. Is it when the service was rendered or when you created the invoice? Near the top, put the label "invoice date." Next to it, add the date that you are creating the invoice. This helps both you and the customer keep track of the time.  You can also include the billing period. The billing period is based on how you set up your invoicing. For instance, if you invoice once a month, that means your billing period is from the beginning of the month to the end of the month, and the services you performed during that period will be included on the invoice.

Summary:
Put your company's name at the top. Add your contact information. Add the addressee's name or business. Include a customer account number. List a unique invoice number near the top. Include the invoice date.