When the wildfire ignited to a massive, Jolly Rancher Green Apple-colored fireball, it was sayonara to the High Sparrow, Loras, Margaery, Mace Tyrell, Kevan Lannister, the heaviest hitters of the Faith Militant, and any of the lords and ladies of the court stupid enough to want to see her publicly shamed. Of course it also had one unintended, and shocking, consequence: sullen, indecisive Tommen suddenly turning sullenly decisive and silently walking off the balcony of the Red Keep.
Before Cersei found out that her last remaining child had been collateral damage in her desperate scheme, she celebrated her victory by pouring red wine on Septa Unella, whom she'd apparently captured and tied down on a table in a torture room. Cersei wanted to hear the woman admit one thing: that she'd enjoyed making the King's mother miserable. Her speech on being a hedonist who acts because things feel good brought such a smile to my face that it looked like I had eaten 17 cheesecake brownies laced with Xanax. The capper was her saying, "Shame, shame, shame," as she closed the door to the cell, sealing Unella in there with the zombie Mountain. As RuPaul would say: she better work.
I'm still not entirely sure how Cersei secured the crown for herself. The court anointed her not just queen, but "Protector of the Seven Kingdoms," which seems like it might have some implications. After the death of Tommen, Robert Baratheon's bloodline has been exhausted, and it's not necessarily the case that Cersei would have been next in the line of succession. One Redditor figured out that if you trace Robert's history back far enough, the only surviving branch of the family tree actually comes back to the Lannisters, which would make Cersei, Jaime, and Tyrion the next in line for the Iron Throne. But since that has yet to be detailed in the series, it looks like Cersei just declared herself the ruler because she'd killed anyone else who might object.
Anyone other than Jaime, that is.