1) Purchase the NCAA rule book. I found the NFHS/NCAA difference book helpful when I first started the transition.
2) Attend your weekly in season local NCAA rules meetings, listen, listen, listen. Do not interject what you know about high school and who you know in high school. Listen and learn.
3) After you feel comfortable with NCAA rules, take NCAA JV games (yes they exist). They are usually scheduled from the local meetings. The JV games will (should) be a mix of veterans and guys in the same boat as you. Be willing to learn and adjust your thinking and take constructive advice.
4) Start attending NCAA clinics yearly.
5) Try to ride along with a D3 crew, get to know them and they will get to know you.
6) Get noticed for your field work during those JV games.
7) When the call comes, you need to say yes, it might be as early as mid week that an official gets hurt and/or needs out of a game. You only get a few chances to get your foot in the door. Once you do, be ready.