Using invoicing systems to make it simple - While you can make up your own invoice or use a free template, it's often easier to use an invoicing system that offers more in depth features. Some invoice systems, like Harvest, offer the option to create estimates for big projects, keep track of multiple clients and send reminder emails when payments are late. Another option is Freshbooks, which offers similar features but includes profit and loss sheets, as well as trackers to help you manage your time.
Across from where you put the recipient's name and contact info, include the details of your invoice. This placement makes it easy to keep track of vital information — for both you and the recipient. You'll want to include: Invoice Number, Date prepared, Payment due date.