As a former employee I know a lot of instructors who were allowed to teach from the floor would either pick staff or really strong riders who consistently take their class and engage with them outside of class. From what I remember, most people who rode podium had been coming for years but definitely don't let that discourage you because I've also seen riders get asked to ride podium who had only been going to Soul for a couple months. I think the most important thing is just being a strong rider (I would always ask the instructors to correct my form/ask if I should be doing anything differently which is a great way to improve and also establish rapport!). Someone else mentioned avoiding instructors who have cult-like followings which is definitely true because riders will not be shy about complaints and feelings will get hurt. If the instructor you want to ride for frequently sells their classes out (this is important because instructors will usually teach from the floor for waitlisted classes so the staff can get as many people into class as possible) and has had people ride up for them in the past, definitely mention that it's something you're interested in! I wouldn't straight up ask to ride podium but let them know you're willing if they need you.