I think you're assuming a traditional conspiracy with all players on the same page, all planned to the finest detail months in advance.. I would agree, that's highly unlikely as there are too many possible leakers and possible ways for the plan to be found out.
They do it the same way they use Antifa and the pedophile LGBQTDFZZZXO freaks to riot and tear shit up and create chaos, and for that matter most Democrat voters, by employing them as useful idiots.
They create an atmosphere and urge people in the direction they want them to go, not with direct conversations and detailed plans, but they encourage certain thoughts and ideas and see if anyone (useful idiot) bubbles up. Then they chat him up on social media, point him where they want him to go by getting him all worked up, maybe drop hints, make it easy for something to happen. It only takes 1-3 conspirators on the ground to pull off what happened in Butler. One well placed guy in charge issues orders that leave a hole open, another at the entrance screening attendees, the rest oblivious just doing their job and going along as ordered. Michigan comes to mind..
OR it only takes one or two at the top to hold back resources, conveniently "forget" to do provide some asset, or issue an order saying "no metal detector screening for this event". You get the idea, they enable by omission and what appears to otherwise be incompetence. 99% of LE/security know nothing about the plot, the plot is fluid, and if the useful idiot pulls it off, he dies, if he doesn't, he knows nothing and can't rat anyone out except some guy named "Kujo" on some online chat.
Its very clear to any one watching that the FBI has become an enemy of the state, a tool for those trying to keep power, a goon squad with nice suits and no accountability.. they are no longer on the side of the American people or the Constitution they swore to protect. I suspect their are other agencies in just as bad of shape and countless government employees that have been riding the gravy train so long they'd do ANYTHING to have it taken away. I know a few.