Have an Answer?
Having been an adjunct professor, my advice to get started is to be aware of the education level expected by higher learning. Some will hire adjuncts with Master's degrees.........MOST COLLEGES AND UNIVERSITIES will only hire those with DOCTORATES (or other "terminal" degrees) for teaching positions. You can go online right now, access lists of colleges/universities currently seeking applicants for positions, and you can see their expected minimum degree level. Be sure to notice the level AND the area of expertise. This search can be quite an eye opening experience!.
Becoming an adjunct professor is a great goal. You asked, "Where to start?"
Without knowing the specifics, I would say you might want to start with a job search. Different universities have different requirements (e.g., doctoral degree, a specific years of experience in the field of expertise, years of teaching experience). It might be good to look at the job descriptions and requirements to identify what you will need to qualify for the position. Once you identify the gap between what you want to do and your resume, you will be able to come up with a plan to bridge the gap.
Best of luck!