Now I will list what Blackbeard and Luffy needed to do to become a Yonko:
The list for what Blackbeard needed to do to become a Yonko:
(The world don't know Blackbeard doesn't sleep, or wounded Shanks's eye or killed 4th division commander Thatch, or that destroyed Drum Island, so it doesn't count)
1- Blackbeard become a Shichibukai by capturing 550 million bounty Prime Ace (Pre-prime Ace stalemated with Jinbe and Base Yamato, and burned WB's hand) by
overpowering Ace's Island Size Flame Emperor, (which hyped Sabo vs. 2 Admirals).
2- Then Blackbeard invades Impel Down, easily defeats the guards,
one-shotted 1000 men Bazooka squad, and puts Impel Down Warden Magellan into coma, and comes to Marineford without any scratch. (everyone knew Blackbeard did this, Big mom's quote proves this).
3- Blackbeard shows people that
he made the most powerful Impel Down escapees his underlings, and even Shiryu, who is Magellan's equal. And declares in the marineford battle that he is no longer a Shichibukai in front of Fleet Admiral and other Admirals.
4- Blackbeard then kills Whitebeard (
everyone knows BB killed WB which is why Bonney and Weevil blames Blackbeard, and wants revenge), and then BB steals Whitebeard's DF ability becomes the first man in history who can use 2 DF powers successfully.
5- Blackbeard then states he will sink Marineford island, and fought with Fleet Admiral Sengoku, later Hero Garp joined him, and they fought them + half of all Marineford forces (other half forces was fighting WB pirates),
Blackbeard sends Tsunami from Marineford Island to Sabaody Island, and damaged Sengoku and Garp (they were covered in bandages after the battle).
6- Around their way, Supernova Jewelry Bonney attacks them, they easily captured Bonney.
7- Blackbeard conquers Whitebeard's new world territories easily according to Jinbe.
8- Then Blackbeard easily defeats the remnants of Whitebeard pirates, who were Marco, Jozu, Vista, Izo, Rakuyo and the rest of the commanders.
Only after all of these, they made Blackbeard a Yonko.
(Blackbeard also invaded Revolutionary headquarters, Baltigo and attack the island while Dragon and Sabo was on it, as well as BB made Aokiji his underling, however these were after Blackbeard become a Yonko, not before, so it doesn't count)
The list for what Luffy needed to do to become a Yonko:
1- Luffy defeats Shichibukai Crocodile, Shichibukai Moria and Strongest CP9 agent Lucci before the time-skip.
(although they covered Luffy's actions in Alabasta they said Marines defeated Crocodile, and hided Shichibukai Moria's defeat, the world only know about CP9 and Ennies Lobby incident).
2- After the time skip, Luffy defeats Shichibukai Doflamingo, then they had a short clash with Admiral Fujitora. Because of his actions, the government decide to demolish the Shichibukai system, and Fujitora wasn't allowed to enter any Marine base until he brings Luffy and Law's heads.
3- Luffy then defeats Yonko commanders Cracker and Katakuri, escapes with saving Sanji. They declared Luffy as the Fifth Emperor, a new Yonko.
4- Luffy then awakens Joy Boy, defeats Yonko Kaido, and saves Wano. His wanted poster shows that Luffy is Joy Boy. Then they finally declared Luffy as fully fledged one of the Yonko.
We also know Law and Kid, despite defeating Yonko Big Mom together, and had 3 billion bounties each, couldn't become a Yonko.
So Buggy either did something bigger than defeating %50 of Big Mom (Law's and Kid's feat), or extremely powerful people, who are stronger than Law or Kid (not together, individually) joined Buggy, so Buggy's crew total bounty surpassed even Law's and Kid's 3 billion.