Don't work upfront and don't stop working.
If you are working for a client, have a set price, ideally hourly if you are starting out. Keep them updated with a time line. Not everyone grasps the amount of man hours you put in and if not informed, they will feel cheated. Once you are more confident with established clients, you can start getting a feel on how to charge for "projects" instead which can help manage your own personal time.
Follow the money. If you are schooled or have had a long interest in visual art ,chances are you know the basics of the adobe suite or at least how to dive in with enough knowledge on how to troubleshoot to make continuous progress. There is a much larger pool of clients who will want digital or vector based graphics for their needs. You already have a knowledge of composition, color theory, and aesthetic cohesiveness which will take you a long way.
Eventually you'll hit some returning clients and that's who you'll want to keep happy. Ideally you'll begin to fall into some who specially like you're style or medium like illustrations for textbooks or magazine. If not, the number one goal is to make money. Keep learning new tools and don't get hung up on one medium or style. Learn to bring over the aspects you enjoy and continue to pursue it in your personal time.
For your one off pieces, if you are able to create a digital marketplace and build potential customers for your oil paintings via your portfolio or social media, sell numbered prints. If people enjoy your artwork they'll want a fair price. It's passive income with enough pieces available and if you find a buyer for the original, that's a nice big payday.
None of these happen overnight though. Get a routine job you know covers your living needs and slowly take that away.