How Do You Make a Nuclear Bomb?
There are two significant hurdles to building a nuclear weapon: 1) acquiring the necessary quality and quantity of nuclear material and 2) fashioning that material into a workable weapon.
Nuclear bombs are generally made according to one of two designs -- a gun-type assembly or an implosion design.
 The gun-type design is the simpler of the two to construct, and works by quickly bringing together two pieces of fissile material, which alone are not sufficient to reach "criticality" -- that is sustain a nuclear chain reaction -- but together form a critical mass.
 An implosion device uses an arrangement of high explosives to create a shockwave that compresses the nuclear material. It is more difficult technically to build but requires less fissile material, and can be built from either plutonium or weapons-grade uranium. The gun-type assembly works only with a uranium "pit" or core. The bomb dropped on Hiroshima was a gun-device; Nagasaki's used an implosion design and contained plutonium.
What is Fissile Material?
 Nuclear material is "fissile" if its nuclei can be split by neutrons in a self-sustaining chain reaction. The splitting or "fission" of each nucleus releases additional neutrons, which go on to split the neighboring nuclei. Each nuclear fission releases a large amount of energy.
 In a nuclear reactor this process is controlled and the energy is used to make electricity. In a nuclear bomb the energy is released in an instant producing a devastating explosion.
Uranium
 Uranium is a metal found in the earth's rock and mined much like any other ore. Raw, unenriched uranium is 99.3 percent U-238 and 0.07 percent U-235. These are two of several "isotopes" or forms that uranium can take, and the two most common isotopes found in nature. Uranium has 92 protons in its nucleus. The isotope number, 238, refers to the sum of the number of protons and neutrons, 146, in the atom's nucleus. U-238 is not fissile and will not sustain a chain reaction.
 U-235 is fissile. Because of the different number of neutrons in its nucleus (143 versus 146), some properties of U-235 differ significantly from those of U-238. In particular, when an external neutron reacts with or "captures" a U-235 nucleus, the nucleus splits or "fissions." This releases energy.
 A chain reaction results when enough neutrons cause the nuclei of neighboring U-235 atoms to split, in turn releasing even more energy. It is this energy or heat, caused by the fissioning U-235 atoms that can be used to generate electricity in a nuclear reactor, or in much larger concentrations, form the core of a nuclear bomb.
How To Enrich Uranium
The process of separating U-235 from U-238 is known as enrichment. One commonly used method involves spinning a gaseous form of uranium at high speed in a centrifuge. This causes the lighter U-235 to separate from the heavier U-238.
 In practice, one centrifuge can only produce a modest amount of separation, so large numbers of centrifuges are employed. A centrifuge cascade gradually increases the concentration of U-235 to various levels of enrichment.
 Highly Enriched Uranium (HEU) is defined by the International Atomic Energy Agency as uranium containing greater than 20 percent U-235. There is no single point at which HEU becomes "weapons grade." Modern weapons in the U.S. arsenal contain HEU that is greater than 90 percent enriched, however, it is possible to make a weapon from 80 percent HEU (as in the case of the Hiroshima bomb) or even 60 percent (as South Africa reportedly did).
Plutonium
 Unlike uranium, plutonium is found in nature in only trace amounts. It is produced in reactors by bombarding U-238 with neutrons. The uranium absorbs those neutrons, decaying into an element know as neptunium, and ultimately into plutonium.

 Nuclear weapons are made with plutonium-239, the most fissile of the plutonium isotopes, though all types of plutonium are considered weapons-usable.
 Plutonium is extracted from the fuel rods of reactors via a complex process known as "reprocessing." Some reactors known as "breeder reactors" are designed to yield high levels of plutonium in their spent fuel. Plutonium can also be used as fuel for a reactor.
 A "closed fuel cycle" occurs when plutonium is reprocessed from spent fuel and recycled for use as a reactor fuel. Reprocessing plutonium on a commercial scale is expensive, and poses both environmental waste challenges and proliferation risks. It is pursued by a small number of countries, primarily Japan, France and Russia, who argue that notwithstanding the risks, their economies could benefit from technologies that reduce their dependence on oil.
For more information on plutonium go here: Joseph Cirincione and Jon Wolfsthal, "Proliferation Analysis: A Plutonium Primer," Carnegie Endowment for International Peace.
What's Needed to Make a Bomb
Approximately nine pounds of plutonium (some experts say even less) and at a minimum 20 pounds of weapons-usable HEU would be necessary to construct a nuclear weapon.
 The point at which a fissile material reaches "criticality" or is able to sustain a nuclear chain reaction is a function of many factors, including the type of fissile material, either plutonium or enriched uranium, and the shape and density of the material. "Fat Man," the bomb dropped on Nagasaki contained six kilos of plutonium.
 Nuclear weapons of more advanced designs use less fissile material but "boost" the yield of a weapon using tritium, a hydrogen isotope.

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