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.
The due date for the invoice is totally up to you, however, most invoicing systems are set up on a 30 day, 45 day or 60 day timeframe. You can also make the invoice "Due upon receipt," where the recipient is required to pay the invoice promptly.