Here's what I posted last time this came up:
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I ran a heist campaign in Spycraft. Here's what you do:

1. Conceive of a location ("the Dwarven Bullion Depository")

2. Draw a floorplan of the location.
>Include bathrooms, service corridors, offices, etc.
>give the map directly to your players at the start of the session.
>don't make it as complicated as pic related because this was a PITA to make

3. Write a blurb about the staff and their schedules. Write about the security systems, if any.

4. Conceive of an INCONVENIENT treasure that the party must steal
>a cask of poisonous grunglor eggs, which must be kept ice cold or they will hatch

5. Have an NPC brief the party on their mission and set them loose.

6. Throw in an unforeseen twist in the third act
>There's a sleeping dragon in the vault?!
>They store all the gold in molten form?!
>The priceless portrait is twelve feet tall?!

7. Once the party has the treasure, throw guards and crossbow bolts at them until they either die or escape.

it's your PARTY's job to actually plan and perform the heist. All you need to do is create the floorplan and arbitrate the rules and NPCs.

Include fun features like skylights and pits.
Give your party funny supplies, like explosives, acid, and invisible rope. They'll find a use for most of it.