Keep in mind whether your industry is one where you invoice upfront before any work has been completed. In this case you'll be detailing the progress of the project as opposed to doing a monthly invoice.
Track your time each step of the way – Time is money, so it's vital to keep track of it. It will also help you learn how long it will take to complete similar future projects. This will facilitate a better work schedule so you can tackle more jobs. Accurate time tracking will add more transparency to your billing and will help clients know what to expect when they need you to work on similar tasks later. Use time-tracking mobile apps so you can clock in and clock out right there on the job and not have to estimate your work hours once you're done.