One of the values of a constitutional democracy is supposed to be that it reduces the impact of the "tyranny of the majority". That is to say, that it reduces the likelihood that a simple majority of people will trivially be able to oppress minority populations. This works by setting up a base set of rules that cannot be violated even if a majority of people in the democracy want to. In order to change or amend these rules, a much higher bar must be met.

However this idea of restrictions placed on a democracy to keep the majority from oppressing minorities is worthless without actual rules. How do you perform the initial population of this set of specially privileged rules then?

Think of it like a problem of bootstrapping trust. A pure democracy cannot be trusted (as a pure democracy trivially allows/enables the oppression of minorities), but non-pure democracies must be conceived and implemented somehow. Can you trust a constitutional democracy that was created by a pure democracy? Arguably no, since the pure democracy cannot be trusted. Can you trust a constitutional democracy that was created by a small set of arbitrary people? Again arguably no, since arbitrary people pulled from the general population wouldn't be any more trustworthy than a pure democracy. Can you trust a constitutional democracy created by a small set of particular people? If you trust those particular people, then perhaps you can trust the constitutional democracy created by them.

People want to trust their government, which leads to them wanting to trust in broad terms the general concepts and ideas that this particular founding group had.

Personally I think it's all shit. A constitutional democracy does not resolve the tyranny of the masses, it only pretends to. Democracy itself is nothing more than "might makes right" combined with a primitive notion of MAD (with anarchy and civil war being the boogieman, rather than nuclear war). Constitutional democracy is good for little more than lying to ourselves to make ourselves feel better about democracy.