Before all that, I'd recommend researching the demand and competition for your course. Udemy's Market Place Insights is a great tool to tell you the level of demand for your topic and the current level of competition.
In regards to Digital Marketing, to be completely honest, it's sometimes most Udemy instructors need to learn to do. I took several courses on Udemy to enhance my online marketing chops. Phil Ebiner and Diego Davila's 36-hour Digital Marketing Masterclass here on Udemy is a good place to start. What you'll find is that some aspects of online marketing take months, if not years, whereas other ones can be accomplished in days.
Additionally, here's a great post on planning your marketing efforts by Lawerence Miller: