How do you activate the Grandmapocalypse?

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.".

You need to have 15 farms, mines, factories, banks, temples, wizard towers, and shipments each, first, and buy the corresponding grandma-related upgrade, such as “Miner grandmas” or “Cosmic grandmas” and then you have access to the Bingo Center/Research Facility upgrade, taking you through 4 other research/facility upgrades and then the “One Mind” upgrade, and that starts the grandmapocalypse․ The catch is that Bingo Center/Research Facility upgrade is 1 quadrillion cookies, and the cost doubles for each further upgrade you go into, capping at 32 quadrillion for One Mind. (It keeps growing for the 4 that come after, though, including Communal Brainsweep and Elder Pact). Find out more at https://cookieclicker.fandom.com/wiki/Cookie_Clicker_Wiki