Trevor mentioned doing some flight training before starting the heist, which I did, but I haven't received word from Trevor about starting the mission. I read online that Michael's flight training and Franklin's shooting range training are optional, but I still have no indication on how to start the heist on my map. (The only marked missions are Lester's, which I'm leaving for later.) Is this bugged?
Lester's first assassination mission done first will advance your game to the point of progressing, and having him call you.
When I was trying to progress the story, I had done all regular and side missions, but left Lester for last. My game had nothing else to do, so I completed his mission. Immediately afterwords, I got a call allowing me to attempt the heist.