Pre-reqs per the latest Standards are 18yo, at least 60 OW dives totaling 40+hours, have Diver Stress and Rescue, have cert or show logged experience in Deep+Nav+Night/LV.
Course is recommended to take 35-45 hours, and is a combo of Classroom/Pool/Open-water. There's always a written test. and you also have to pass the Science of Diving final exam... and of course pay the Pro fees and get insurance coverage (which is generally thru the shop policy for SSI, in the US at least)
Specific course format of the practical parts can vary based on the instructor, but safe to assume you'll spend some time as part of actual classes, or at least adjacent.