Q:

How could a single individual warn the following civilization after the fall of our current one?

Reviewers: reformulation according to this is done if the post is not okay yet, suggestions are welcomed in comments
The limits of the individual:

has no access to the mass media.
has the average financial resources what a commoner has in a Western state.

The mindset:

knows that some social apocalypse nears.
knows that is the result of the social manipulations of an X group.
knows that it is unlikely that his/her civilization will survive.

Here I would show an analogy: in the last centuries of the ancient Roman Empire, it was already more or less known, that with time, the Empire will fall. It was a significant change in the mindset; some centuries earlier the "common knowledge" was that the empire will last forever. The ancient Romans weren't dummies: they didn't know a lot, what we know, but they already knew the world map and their history.
Now back to the present. So, there is knowledge, that he/she belongs likely to the last generation of his/her people, and to the last generation of his/her civilization.

The apocalypse won't be a sudden, catastrophic event, rather a long process, a protracted series of small events, and a continuous decrease of the living standards and all significant indicators what could show the development of a civilization.
Rome or Asimov's Trantor was dying centuries long, the time scale of the fall of the civilization seems some decade long. And the historians of the future will see the era of the dying of your civilization.
However, contrary to Asimov's Trantor, or to the ancient Roman Empire, the cause of the fall is this time not a complex structure of social, demographic and political mechanisms. This time, the cause of the fall is a

very conscient
very determined
very manipulative
very well-organized

action of a small, well definable, but anyways nearly invisible group. We could call them "group X". Contrary the conspiration theories, the difference is that this "group X" is really existing (or at least the individual thinks they are).
It seems likely, that some centuries later, a new civilization, a new highly developed era will likely grow on the remains of the current world. It seems probable that this "group X" will be likely active also at this time.
The individual has some decades lifetime back. He/she can, and want to sacrifice these decades to somehow warn this following civilization.
How could he do this?
Note: actually, it is a pragmatic thing - the goal is to reach the possible largest effect in this far future, as the effect of this warning, even if this effect is small on the global level.

About "group X": it is a very well determined group, working as a loosely coupled social network connected by various (sometimes contradictory) ideologies since at least millennia. Nobody knows, where are they from, nobody knows, why are they doing this. Their most characteristic properties:

they don't consider the human as "real" humans, only themselves
although they are probably humans with ordinary human skills
they are very sensitive to the blood loss in their group
they consider the rest of the humanity as dangerous, semi-intelligent animals
they have a cultural heritage preferring soft power, background jobs
they have a strong urge to remain invisible
they like to "use" others for the background, occupying influential positions (mass media, political decision-making and so on), or controlling people in these positions, mainly by their immoral wishes
they are capable and willing to plan in centuries and execute these plans.

A:

Your only hope is to bring this to the eyes of the governments because you yourself can not hope to ever deal with a group of people with more power than you. I would say if you have proof of their existence and there isn't a greater conspiracy, simply bringing attention to it and the police/FBI or w/e will eventually deal with them. 

A:

Leave "Time-Capsules" with as many young honest people as you can. 
Choose promising youths and entrust them with your knowledge, form a Resistance, a secret group who will keep their eys and ears open and will not be afraid to give their lives for the truth. 
Be prepaired to die for the truth, to fight with all your strength, physical, mental and emotional, to ensure the results you wish for.
There's only so much you can do completely alone.

