Android needs to be built for the specific hardware, you can't just download a generic android build to any device.
Unfortunately what this means is that unless someone has already done this for your device type - modified the low-level software so that you can load android, and built android with the drivers etc needed for that particular hardware - the only way is if you learn how to do this yourself. This is not a trivial undertaking.
Without knowing what device you have, I cannot say more. But I think that most likely the answer is that you can't.
Sorry.