Interviewee Stage

The easiest and best way is to reach out to the ROTC unit and request a interview. Some have links on there websites, but just about all have the contact information for the units. Choose a unit and contact them about what you need.
My DS had a list of PMS at different area colleges. They only listed a name and phone number. He tried several unsuccessful phone calls, and finally looked up the person on the college website, found an email, and interview is scheduled.
My kid was aggressive, but did not want to be the first to go. He did his interview at ASU a few weeks ago and is hoping for a decision from the first board like his brother got.
It's often easier to reach out to ROTC units from all branches by going through their college/university website contact information.

Often, the unit leadership staff/cadre are more easily contacted through their ".edu" school email addresses rather than through their ".mil" email addresses. We've had a few instances where our personal email adresses get bounced from ".mil" email servers because of tight security parameters.
I would recommend doing it at your top choice school if that is logistically possible. If you have any previous contact with a ROTC Unit that is also a good place to start as they are at least familiar with you. If neither of those is possible, I would contact the closest unit and arrange for an interview. If it appears that they are not very familiar or receptive to that process, I would seek the next closest Unit that is.