One of the easiest ways to start a sideline business is to monetise an existing skill. If you have drawing skills the custom portrait and caricature business is something to look at.
Many people and businesses would like to have portraits created, whether for personal use, as a gift or to use in an editorial.
This business creates a custom portrait and caricature from a photograph, it is something you can do as a sideline after hours and in your own time.
Marketing & selling your services
The most obvious question is how to find clients. There are platforms like fiverr that connects buyers and sellers of these types of services. You can also advertise via social media such as Twitter or Instagram and even on your local Gumtree.