You can't chemically separate the various isotopes of an element as they are identical.
The only way is to use the fact that an atom of U-238 is faintly heavier than an atom of U-235. It's the U-235 you want in a very concentrated form, as it's fissile
So the you need a physical, mechanical process. That involves turning your mixed Uranium into a gas, and then using a centrifuge to spin it at extreme speeds to cause the ever-so-slightly-heavier U-238 to move towards the edge.
As with a lot of things (e.g. moon shots), the science is A-level. The engineering is insane.
Be careful what you write. The US has a doctrine of "natural born secret". Which makes certain secrets illegal to know, even if you discover them independently.