Comment by Brian_K_White

By doing it.

Individual cpu instructions, even of a crude old 8-bit cpu with no embedded minix os like today, are both simple enough for a human to manually understand what they do, and useful enough to build crude versions of useful things like an editor, interpreter, or compiler.

You can write a forth-like language or even a c-like language starting from individual cpu instructions that a human can read, understand, and write totally manually, and then use that to build up rapidly all the way to a full modern desktop.

If you were really paranoid about the very act of the initial typing-in, there are any number of ways to store data in a totally brainless eprom or record it to tape or something, and examine it with nothing but some leds, no cpu at all, to verify the bytes are the bytes you want. And you only need to do that for a pretty small number of initial bytes. After that it's all just regular source code which could be written on paper.

Bootstrapping is only an inconvenience problem, not a real problem.

It's not convenient for most people to assemble some bytes into some storage medium and then verify them without simply using a normal untrust-able computer to do it. But it's no problem really if you had some reason to be that careful.