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.
Make sure to include a column for each of the following: description of work or services, date worked, quantity, rate, hours worked and subtotal. This makes it easy to tally up the total when you list out multiple projects or tasks you've completed for the client.