The hard part is getting enough enriched uranium. it's chemically the same as all other uranium ore, and the only difference that you can use to separate it is

The hard part is getting enough enriched uranium. it's chemically the same as all other uranium ore, and the only difference that you can use to separate it is a 0.85% mass difference of the atoms. Once you have the uranium extracted, the bomb itself is trivial, as far as these things go. You need some knowledge about how it works, but the actual parts are simple enough. In fact, you could probably put together one with steampunk technology without too much trouble if you had the uranium