PSIA has 9 different divisions that each have slightly different processes for certification. Most (I'm not sure if all) require you to be an instructor with a ski school in order to become certified. People usually join the division covering their geographic area either where they live or where they work. Most divisions have online study guides that will help to prepare you for the exam (e.g. the two that Kogu has referenced). Do you currently teach at a ski school? Where do you live?
Years ago the path from beginner to expert started with wedge turns, followed by wedge Christies and ended with parallel turns. Now we teach some beginners to make parallel turns from day one. This was the main impetus for the stepping stones concept. But it is larger than that, Within the overall concept of guest centered teaching is the idea that everyone learns in their own unique way and will learn best when they follow the path that is right for them. Some people may need to take the shortest path across the water while others may need to step on every stone and still others may need to step on the same stone multiple times. It's the job of the instructor to know all of the stones and help to choose the best "next stone" for each of their students.
PSIA has 3 stones (levels of certification) on the path to becoming a great ski teacher. The level 1 certification is designed to make it easy to get you started on that path. As you get more experience, you'll find that there are thousands of stones in the ski teaching river. The PSIA stones are like a map to help you choose your own path across the river. Good luck on your journey!