First of all, building a thorium reactor at home is neither legal nor safe. But, if you’re an absolute mad lad and have no strain about the upshots, this is how you do it.
A proper thorium reactor would need a critical mass of principal fissile material: uranium-235 (U235), plutonium-239 (Pu239) or uranium-233 (U233), to run so that you can breed thorium-233 (Th232) into U233. Mind that the process of breeding U233 from Th232 (thorium fuel cycle) would spew out huge amount of radioactive radiations enough to make the neighborhood uninhabitable.
The next thing you’d need to do is – remove the irradiated thorium targets and keep them for some time while the protactinium-233 (Pa232) disintegrates to uranium-233. Then you’d need to extract the U233 from the thorium targets and reprocess it into fuel.
Now assuming all the required significant industrial infrastructures, and licencing problems are met to properly design fission reactor, it’s time take a shot at building something that is proximate to X-10 Graphite Reactor – the world’s second artificial nuclear reactor made during World War II as part of the Manhattan Project.
Fuel reprocessing is the only part you’d find to be most challenging because the kind of fuel reprocessing needed in the thorium-uranium fuel cycles requires a containment chamber (hot cell) which is robust enough to shield highly radioactive fuels. Also, dissolving uranium-233 out of thorium would probably yield some acids and a radioactive waste stream.