How do prions that are ingested cause Creutzfeldt–Jakob disease and Mad Cow disease?

You mean after digestion? I do know the prions enter the bloodstream because it is commonly thought that calves that drink the milk from their dams that are infected with prions causing BSE (bovine spongiform encephalopathy) are also liable to get BSE. I think the prions get to the brian through the bloodstream, as don't forget that blood vessels and capillaries have to supply the brain too in order for the brain to work like it does. Now why prions settle in the brain and spinal cord is something I believe that scientists are still trying to figure out, but it could be that the prions that are genetically and biologically structured so that they "fit" in with the proteins in the brain. But that's my own guess, which could be wrong.