Things you need:
• $10 million
• 150 acres of open space
• A workshop
• A lathe, a furnace, a surplus artillery gun
• A talented group of engineer types who know their way around weapon design, metallurgy, ballistics, electronics and physics.
• Oh, and that nuclear stuff: plutonium or highly enriched uranium (HEU)
MAKE IT!
There's two methods in triggering a nuclear bomb: an implosion device or gun assembly. An implosion-triggered fission bomb compresses fissile masses together through an explosive charge. That sounds crazy, cool and crazy cool but the gun assembly is easier to make, and was used in Hiroshima. Pro tip: only use HEUs in gun assemblies. The hot market to grab HEUs is in Russia, where they don't keep strict records of their nuclear material and have tons of it lying around from their Soviet days.
If you shoot one piece of HEU at another, you take out Toledo. The process is a little like getting a ball of HEU, removing the core as you would an apple's, bolting the pitted end to the muzzle of an artillery gun, then firing the core back into its center.
In order to do that, you need to melt the Uranium at 2,069 degrees Fahrenheit. Once properly melted, you place it in a lathe for shaping. Then lock and load an artillery gun and boom, bang and out. You're a terrible human being.
Sure, maybe it's not that easy but the world needs to keep better track of their nuclear material. If the wrong hands—terrorists, North Korea, or horrifically evil but rich folks—get their hands on the plutonium or HEU that is floating around in the world, we can all say hello to Nuclear Warfare and goodbye to life.