[MODERATOR: reference to a deleted post edited out.]
The explanation to build a nuclear bomb is really not that difficult if you understand the concept. For a gun type, all you're doing is take a mass of fissile material and set off an explosion which shoots the material into a larger mass of material. This creates a super critical mass which sets off a chain reaction that fissions the majority of the fissile material at once creating a huge explosion. This information can be found on wikipedia. I'd bet there's much more details available on the dark web.
The real problem is getting the material to actually do it. Natural uranium is 99% U-238, .7% U-235. To make a bomb you need at minimum 20% U-235 but ideally you want over 90% U-235. To enrich uranium requires putting it through a massive centrifuge to separate the isotopes and you need thousands of them to gradually step up the enrichment. It takes years of this process to stockpile enough material to make the bomb.
So say you want 5 lbs of 90% U-235 to make a bomb you'd have to process something like 10,000 lbs of uranium. Not exactly something you can do in your backyard.
If you're making it with plutonium it's even more complicated as you must use a breeder reactor to first create the plutonium
This is not something any country can do in complete secrecy. Just like we discovered North Korea building one and we discovered Iran trying to.