This question already has an answer here:
I'm searching for a reference of an undecidability proof that is as simple as possible and starts "from scratch".
With "from scratch" I mean that it does not use some other undecidable problem to prove some undecidability (which is the usual case), I cannot wrap my mind about how proving undecidability that way (without a previous proof) could be possible.
This question may be inspiring: An example of an easy to understand undecidable problem
Also, I know this is probably not very objective, but it is important to me, it should be something as simple as possible, hopefully enough so that even I can understand it.