This goes without saying. People aren’t going to know that you’re offering a class if you don’t tell them. Send out flyers or post a Facebook Event. Be sure you’re clear about the project you’ll be doing with them and what the cost will be, and if you’re offering anything else to people.
Also, be sure to let them know the level of expertise the class is for. If you are showing people a more advanced task that a beginner is going to struggle with, then the beginner artist is likely not going to enjoy your class and won’t recommend you to anyone. Word of mouth is going to be very important in this type of venture, so you need to be very clear about what you’re offering.