How do I activate the Grandmapocalypse in Cookie Clicker
You can trigger the Grandmapocalypse by purchasing the "One Mind" upgrade from the Research Facility.
If you don't have a Research facility yet you can unlock it by having the "elder achievement" (have at least 7 different grandmas).
Once you have the research facility you will have to keep buying the upgrades that it unlocks until the "one mind" upgrade becomes available, which should be the 6th one. Each upgrade takes about 30 minutes to research and become available. Note that you need to purchase an upgrade before the next upgrade begins researching.
Another way to activate it is to first make Grandma hate you. And to do that you just need to make enough cookies and then she'll start saying things like, "It begins", or "We rise". When she starts saying that, you know it is getting close to Grandmapocalypse.
If you sell a Grandma, it gives you an achievement. "Sell a Grandma" and on the bottom right corner it says, "I thought you loved me", and then you will see messages from grandma saying,"It thought it could get rid of us, the nasty little thing.", or "It thought we would go away by selling us, how quaint.".
But i'm no expert; I mean, I've only made 60 Trillion cookies and other people have made 60 Septillion or something. Sorry for the many grammar mistakes that might trigger you, I'm not very good at grammar; but I tried my best.