Bruce Schneier has an eight-step plan—though he recognizes Step 1 might not be possible:
Since the 2016 US presidential election, there have been an endless series of ideas about how countries can defend themselves. It's time to pull those together into a comprehensive approach to defending the public sphere and the institutions of democracy.
Influence operations don't come out of nowhere. They exploit a series of predictable weaknesses -- and fixing those holes should be the first step in fighting them. In cybersecurity, this is known as a "kill chain." That can work in fighting influence operations, too -- laying out the steps of an attack and building the taxonomy of countermeasures.
Step 1: Find the cracks in the fabric of society -- the social, demographic, economic, and ethnic divisions. For campaigns that just try to weaken collective trust in government's institutions, lots of cracks will do. But for influence operations that are more directly focused on a particular policy outcome, only those related to that issue will be effective.
Countermeasures: There will always be open disagreements in a democratic society, but one defense is to shore up the institutions that make that society possible. Elsewhere I have written about the "common political knowledge" necessary for democracies to function. That shared knowledge has to be strengthened, thereby making it harder to exploit the inevitable cracks. It needs to be made unacceptable -- or at least costly -- for domestic actors to use these same disinformation techniques in their own rhetoric and political maneuvering, and to highlight and encourage cooperation when politicians honestly work across party lines. The public must learn to become reflexively suspicious of information that makes them angry at fellow citizens. These cracks can't be entirely sealed, as they emerge from the diversity that makes democracies strong, but they can be made harder to exploit. Much of the work in "norms" falls here, although this is essentially an unfixable problem. This makes the countermeasures in the later steps even more important.
Also unfortunately, most of the countermeasures require informed and conscientious political leaders. Good luck with that.