A professional header: The first item on your freelance invoice should be your business name, or your full name, in professional (and easy to read) font. The font size should be a little bigger than the rest of the text on the invoice, and possibly even bolded for emphasis. Next you'll want to include your contact information: your mailing address, phone number, email address, website, etc., right underneath your business name.
Due Date: The Invoice Date plus Payment Term Days give you the Due Date, which is in plain English and easier to understand. Having a Due Date on the invoice is a tactful and professional way of making it clear to your customer when they should pay. Bill To: Who this invoice is intended for and their contact information. Product or service details: Specify Description, Quantity, Rate, Amount and Subtotal. The more details you include on the invoice, the better. This way, your client will know exactly what they are paying for upon receiving the invoice. Make sure that your client can understand every item, so that as soon as they receive the invoice, they can pay you instead of asking questions and delaying the process.