We found that regular communication with sons' vetted list of schools (both baseball coaches and admissions counselors) was critical and showed each school my sons' "demonstrated interest".
My boys did not do this haphazardly, rather, they followed a regular communications plan based on meaningful info to be communicated.
The pool you're swimming in has something to do with this, i.e., what schools are on the player's vetted list and when that school begins recruiting. Generally, for the mass of players who aren't draftable or Top25 program talent, the Fall of Junior year proved to be optimal timing for an introductory email for my boys (2012 and 2016) and the kids I work with. I just checked my 2016's #1 school, and he first connected with the coach on October 30th of last year.
Follow that with (written, phone, email, etc.) communications. Happy to discuss what else my sons communicated and how they did so if you're interested.