Make (and study) art X hours per week. Research art marketing techniques Y hours per week. Implement art marketing techniques Z hours per week.
So, for example, make art 30 hours per week, research art marketing techniques 5 hours per week, Implement art marketing techniques 8 hours per week. (Research your art medium too so you level up as a fine artist.)
The key is doing this week after week. Because then your art doesn't just get better, but your grasp of how to sell art gets better and you actually take the time to do art marketing. Also, since you keep researching marketing techniques, you don't stagnate. There are myriad techniques for both in person vending and various forms of online sales (even connecting with people to sell your art for you). You can do whatever you like. So, really it's like two different jobs that together make you an income, making art, and leveling up at selling it.
NO JOKE, I surveyed multiple successful artists and they all said basically this same thing. Ever since I started doing this, my average income has slowly risen. It really works. Discipline, dedication, and open mindedness are the keys in my opinion. Good luck! :)