text: Depends on whether you want to use fusion or fission, and on how large of an explosion you want.  Since for fusion, you need to much energy to get it started in the first place for you to be able to make it small, fission is the way to go.  You can make a solution combining uranium with fluorine and then diffuse the solution through long thin tubes.  The uranium 235 (the type that goes boom) will pass faster through the tubes and after a lot of time, effort, and money, you can get a solution of Uranium with a high enough concentration of the 235 isotope to be able to detonate it.  To prevent detonation, you want to use a moderator that will absorb neutrons before they can induce detonation.  Cesium works well for this because of its high density.  Set up a remote control that can remove the moderator and you have a reaction that accelerates exponentially (fast) and releases huge amount of energy.  Boom.  \n\nWhile bombs can be used for good and constructive purposes, I doubt your motives.  Regardless, the construction of atom bombs is public knowledge.  I believe that you would find obtaining uranium to be the easiest step, and that is not saying much for the easiness of the rest of this type of process.\n\nAs a side note.  The USA is one of the last countries still using diffusive methods for enriching uranium.  Most countries use centrifuges that are more efficient.