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.
Next, you need to specify your payment options — whether you prefer to be paid with cash, a check, a credit card or a service like PayPal. Some companies will offer direct deposit, if you do regular work for them, but more than likely you'll have to send an invoice to request payment every time you complete a project.