It's not rocket science, and there are other methods available as well. It does take a lot of energy and time though. Probably easier for a low tech setup to

It's not rocket science, and there are other methods available as well. It does take a lot of energy and time though. Probably easier for a low tech setup to just bombard U-238 with thermal neutrons. Once you get enough U-235, just use beryllium to reflect the neutrons back in and add a moderator(water, graphite, etc). Just don't be anywhere near it when it finally goes critical.