I agree that the question is too broad. How big a museum? How up to date is their security? Do they have something "priceless" or are they some local history museum with no rare works of art, etc? Is there an "inside man" or do the crooks not have or not need the help of anyone in the museum? How expensive/well known is the thing they want to steal? Does it have any special security on top of the general security? Another way to look at it: How important is the actual museum break in to the plot/story? I mean, is this a "heist caper" where the story is all about the gathering of the team, the recon of the target, and the exact methods of how they break in, or is all that not really significant because the story is about something else and the plot just happens to require something from the museum? If it's a heist story, then you'll want more detail on the security and how they bypass it because that will be a large part of the book. But, if the story is really about what they do with the thing that's stolen, and the details of how they got it don't matter, then just write it so the security is poor, they bribed or coercied someone to help them, or found an easy way in like a unsecured window. (Which has actually happened in real life, btw) Right now your basic question of "how to break into a museum" is as broad as "how bad is a car crash?" Define what you need story wise a bit and you'll get more insightful answers.