Transmission begins immediately after I Walk the Line ends, although you can turn down Brigitte’s offer to run around and do other things in between the two quests, if you wish. Once you inevitable end up accepting her offer, however, Transmission will really get started.
After the time skip, follow Brigitte to the “crypt”, engaging in another walk and talk. What you say doesn’t matter, and your journey will end with you jumping into a tub of ice, after which you’ll end up in cyberspace. Talk to Brigitte and after some chatter the quest Transmission will be suspended for another Johnny segment, Never Fade Away. Just get on stage and grab the microphone to make it official.