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.