What did Cersei and Qyburn have to do in order to pull off what they did?
In Game of Thrones season 6 episode 10, they
use wildfire to blow up Baelor’s Sept with Margaery, Mace Tyrell, Loras, the High Sparrow, the Faith Militant and all those people present during the trial of Loras and the intended trial of Cersei herself.
So how did they set it all up? How did they do it undetected? And where did they get the
wildfire
?
The Mad King aimed to destroy the city when Robert Baratheon started to prevail in the battle. Those barrels of wildfire were there with his command.
The Mad King once said:
"Let Robert be the King of ashes."
With Jaime killing the pyromancers, people aware of the full extent of this plot are gone.
The wildfire caches were later found by other people but removed by the alchemists guild. In fact, some of those caches were used by Tyrion in the Battle of Blackwater Bay.
In Season 6: Episode 8, we remember Cersei talking to Qyburn
"Your Grace, that old rumor you told me about. My little birds investigated."
"And? Was it just a rumor or something more?"
"More, much more!"
They were talking about the caches of wildfire under the Sept of Baelor. Detonating them is as you have observed in the show.
SPOILERS
The wildfire was hidden there for many years. During his last moments, Mad King Aerys Targaryen II was planning to blow up King`s Landing with wildfire , which is the reason why Kingsguard Jaime Lannister killed him (S03E05- Kissed by Fire), and his last words were "Burn them all!"(S01E03, Lord Snow, S06E06, Blood of my Blood).
He secretly gathered thousands of jars of wildfire under the city, with most of them being placed under the Great Sept of Baelor, the Dragonpit and the Red Keep (A Storm of Swords- Jaime V) .
When Tyrion visits Hallyn the Pyromancer to get wildfire for the Battle of the Blackwater,Hallyn says that there are about 7.840 jars of wildfire with about 4.000 coming from the Mad King's failed plan, and that he is positive that he will make 10.000 before the Battle (ACOK, Tyrion V).
We don't know how many of the jars were used, but it is impossible that Tyrion used them all.Therefore there are still some jars under King's Landing.
Cersei must have known somehow about the wildfire. She was the queen, and in the show she was the first who thought wildfire could be used in the Battle of the Blackwater (S02E05 -The Ghost of Harrenhal).
In S06E08- No One Qyburn tells Cersei :“That old rumor you told me about — my little birds investigated” to which she replies :"Tell me, was it a rumor or something more?" and he tells her: "More, Your Grace. So much more."
It's obvious now that they were talking about the wilfire.
We don't know exactly how Cersei planned this to so much detail, so these are the only things I can tell for sure. The rest of this is shown in the episode itself (S06E10- The Winds of Winter.)
