This build is a smart kovarex and nuclear fuel cell factory. Simply replace the combinators with the listed inputs and give it some time to start up. Having a few hundred U-238 helps. However - once the factory has enough U-238 to run all of the centrifuges at once, It will maintain an optimal level of U-238 without overproducing or underproducing.
The bottom Input is for unfiltered output from uranium ore centrifuges, and will automatically add any U-238 to the system. Simply supply it directly from the output of some centrifuges.
An important point to note is to only remove U-238 from the system using the dedicated belt next to the fuel cell assembler. The stack inserter will ensure the factory stays at an optimal level of U-238 and will prevent you from losing the buffered amount of needed U-238