First ring, personal data, family photos,....
First hidden "volume" - banking data, credit card info,...
Second hidden "volume" - dick pics, some embarrasing porn
Third hidden "volume" - atom bomb plans or whatever you're actually tryng to hide.
A nuclear bomb just has to bring a critical mass of material together. For Uranium 235, that's about 104 pounds. You typically do it by splitting it into 4 pieces, and stick something like dynamite to each one so that when they go off it goes together and goes boom.
A hydrogen bomb is an ellipsoid made of aluminum foil with a nuclear bomb at one focus and heavy water at the other. The flash from the nuclear bomb hits the foil, and refocuses on the water that makes for a substantially larger boom.
This is all well-known. The trick is getting a sufficient supply of enriched uranium. The principles of how to do that are described in https://fas.org/issues/nonproliferation-counterproliferation.... Note that operating with elemental fluorine is dangerous to your health, and the necessary centrifuges WILL be spotted by US satellites, who are unlikely to be sympathetic with your desire to build a nuclear arsenal.
The fourth ring or fifth ring is where it starts getting interesting.
I could build Twitter in a weekend!
(It's never as simple as there executive-level "big picture" makes it sound...)
In theory, you have a rough design for an assembly.
In practice, there is still more than enough knowledge missing to keep you blowing holes in/irradiating the countryside for quite sometime to come, and that's being generous, and granting no one actively interferes with your efforts at building a cache of recreational McNukes.