It sounds simple but you've got two roadblocks to overcome. The first is "sufficiently enriched uranium" which is expensive to obtain. The second is "fast enou

It sounds simple but you've got two roadblocks to overcome. The first is "sufficiently enriched uranium" which is expensive to obtain. The second is "fast enough", since if you don't assemble the material fast enough and hold it together for long enough it will blow itself apart before there is a chance for the chain reaction to build, and you will have a dud (at least compared to the intended yield). To get around this problem you will probably need some fairly advanced shaped charges and a properly designed tamper. This is what the "nuclear secrets" cover. Obviously it is possible to derive this information for oneself, but it's non-trivial to say the least.