Not really a hard achievement in terms of skill applied - especially After you've done with the Civil War. But it actually takes AGES to get 6 coups.

Well, the achievement description says it needs 5 coups, but many people reported already that they needed to make the 6th coup to unlock this achievement.

Here we go:

1) Win the Civil War, either side is ok, but the main goal is to get into some major faction - Axis or Allies. USSR usually quite defensive, so you'll probably not go too far with the Comintern.

2) During or after the Civil War, you need to build a spy agency. When you land into some fraction, make yourself the Spy Master of it. Now, you will have enough spies to start some coups. The more different countries jump in the same fraction, the more spies you'll get, but 3 is enough for the start.

3) Now you need to search for some minor country with a ruling political party different from yours. It should be more than one province large, but not too big, AND, with as little stability as possible. We need less than 70% to start a coup, but the lower - the better. Then, we need 1 spy to maintain the spy network in the target country, and other spies to do actual indoctrination and stability ruining. After you have about 30-35% popularity for your party and less than 70% stability, you can trigger the coup preparing process, which takes almost a year to proceed. With about 45-48% party popularity, the coup will surely be more successful - BUT You Don't actually need them to succeed, to earn the achievement. Just start the trouble enough times, in different countries, and you'll get the badge eventually.

PS: If you'll hype the ideology in the targeted country over 50%, you'll throw the current government Without starting a coup. Generally, a much cheaper and better option, both time- and resources- wise... BUT not for this achievement, alas.