First you need a certain amount of weapons grade uranium-235 (U-235) or plutonium-238 (Pu-238). That means that it has to be something like 95% pure. (The stuff that is used in nuclear reactors is only 5% pure.) Then you have to a cause this radioactive material to achieve critical mass. (Critical mass is the the situation in which the fission chain reaction goes out of control.) There are two ways of achieving critical mass. The first way is to have two separate pieces of nuclear material that by themselves, don't constitute a critical mass, but do when put together. When you do bring them together, the reaction occurs and the bomb detonates. In the bomb that was dropped on Hiroshima, Japan, they did this by mounting one piece of U-235 across the muzzle of a sawn off artillery piece and the other chunk was fashioned into a bullet that was fired into the first chunk at the time of detonation. The other way to achieve critical mass is to start with a single piece of fissionable material that is not quite big enough to achieve critical mass. You shape this piece into a sphere and completely surround it with high explosives. You then detonate these explosives. This explosion crushes the sphere causing it to achieve critical mass by increasing it's density rather than by adding more material. Note, you must make all of the explosives detonate at precisely the same time. This requires a multiple detonators that are timed very precisely. If the timing is not just right, then you shatter the ball of fissionable material rather than crush and critical mass is not achieved. This is probably why this is the preferred design for nuclear bombs today. It is significantly more more complex than the first design, but it is also much harder to detonate one of these bombs by accident. (If a nuclear armed bomber were to crash a bomb of the first design would be much more like to detonate than one of the second design would.) Further note, I have not written anything here that hasn't been public knowledge for decades. I am not a nuclear scientist nor have I studied nuclear physic beyond what was required of the three mid-level physics coursed I took as part of a degree in geology. So, I am not revealing any national secrets here. The big problem with actually building a functional nuclear bomb is not the design. It's obtain enough weapons grade U-235 or Pu-238. It very difficult to get these isotopes of these elements in sufficient quantities and purity to achieve critical mass. For the decades since the first nuclear bombs were built, this has been the thing that has kept all but a handful of countries from becoming nuclear powers.