I have been asked this question several times. When I first started officiating football, I also signed up for basketball. I did both at the high school level. The procedure to become an official is here:
Ohio H.S. Sports Official
After working 2 years in each sport, I asked our Chapter president how I could advance. He told me to get either higher marks from coach's evaluations or attend a camp clinic. I decided to go to a 4 day camp that was held by 4 clinicians from the NCAA and 2 clinicians from the NFL. It was held at Lakeland Community College in Kirtland, Ohio. I was living in Ohio at the time.
After the camp was over, I was offered a job officiating D-I football games in the Big Ten and the American Conference, which I accepted. I made both conferences well aware that I would not be available on Mondays through Fridays due to my work schedule. That was OK with them and they thanked me for my honesty and letting them know well in advance of my closed days. I wasn't interested in doing college basketball games at the same time as football games.
After 4 years of working The Big Ten and the American Conference, I started getting offers from the ACC and the Sun Belt Conferences, which I accepted. After another 6 years of working D-I football games and the NFL officials were talking about going on strike, the NFL offered me a job. I told them I would only be available on Friday nights, Saturdays and Sundays. I had my lawyer read the contract and after making one adjustment. I signed it and it was at that point, I became a part time, fill-in NFL Official on Saturdays and Sundays only. That meant, any full time NFL Official that was unavailable for Saturday or Sunday games, I would be be available to work those vacancies. I signed the contract along with Commissioner Pete Rozelle.
The worse game I encountered was the fight between Cortland Finnegan (Tennessee Titans) and Andre Johnson (Houston Texans) . Both players removed their helmets and a fight broke out. Both players were fined $25,000 and neither player was suspended. Team members from both sides and the officials helped to stop the fight. It was a real bruhaha of a fight. We had several other little scuffles, but this fight was a real knocked down, dragged out fight. I took one hit to the back of my head, which was an accident, I think.