If it's the PCs pulling the heist, you don't really have to write much, especially if you're any good at coming up with PCs and small details on the fly. You need to know what they are going to steal, where it is, and how & by whom it's protected. You might have to provide a motivation to the PCs to get them to steal it and/or dealing with fencing it.
For me, personally, the only prep I'd do is for the object they were to steal (easy if it's cash or something like gold), maps of where it will be (architecture books and the internet are good sources of easy maps), a decent idea of the security systems and a few dozen NPCs. I might not even do that until I've thrown the bait to the PCs and seen if they take it.
Everything thing else would be determined on the fly while observing the players RPing their PCs planning the heist. Because honestly, in an adventure scenario like this, I really don't see a way to foresee everything the players might do and pre-writing too much could tempt a GM to railroad them.
I would probably sit down before hand for a couple marathon sessions of heist movies like the Italian Job (both versions), Ocean's 11 (both versions), the Ocean's sequels, Rough Cut and even some comedies like the Pink Panther, etc. Whle watching these, I'd take notes on good ideas for security systems, NPCs, etc. By the time I got to the table, my entire "adventure" would probably consist of a couple of pages of scribbled (and probably incoherant) notes, a few pregenerated NPCs and a map or two.