This is the method I used to unlock, I had historical history turned on
To start a civil war you must go down the monarchist focus tree, which causes the communists and republicans to go to war with you.
When you start the game immediately research "King's government", then once the election is complete research "The right to rule". This will cause a civil war. This will give you more time to win the civil war.
I found that when the civil war starts the enemy controls Peloponnese and Central Macedonia. So when the game starts, build land bunkers Korinthos so when the civil war starts a single division can defend that bottleneck while the rest of your troops clear the north. Upon the north being cleared those troops can go south and push to the capital.
On my successful attempt, I dispanded my airforce so when the civil war starts I have access to them by redeploying them.
Finally, I dispanded the horse division and around 5 divisions which had low equipment. As when the civil war contains more division its a slow and full attrition and you don't have long to before 1937 comes around. As well this makes the enemy have fewer troops, which allows easy circling.
Last note, as both sides are weak and under-equipped be prepared for it to take a while for a battle to be won.
Be patient and good luck!