I took an introductory astronomy course during my senior year of university. In that class, I learned about these things called neutrinos—they're these practically mass-less particles that are extremely hard to detect. Basically, this guy named Wolfgang Pauli suggested in 1933 that there is a fourth type of elementary particle in addition to protons, electrons, and neutrons. These other particles (antiparticles) would be neutral, have zero mass, and "carry away" the energy that was noted to disappear during certain nuclear reactions (source).
So, anyway, I was mindblown learning about this. And then I thought, "Oh, Neutrin-Os. That would be a hilarious cereal brand. And I could do a super campy 60s thing with the visuals." Ta-da!
My process involved studying scans of 1950-60s cereal boxes and finding the common threads. Things I noted were mascots (which take up a large portion of the composition), photographic or illustrated representations of the cereal (important because the boxes were opaque—no budget for plastic windows), and fairly prominent gimmicks like prizes, lotteries, or in-box games.