Do not wait until the real civil war.
Prelude
instead, what you must do is that you add a Communist Revolutionary to your government slot, Researching the focus Political Effort can give you a headstart.
Train your troops, you will need them for later in this civil war.

Wooo its coming together hopefully
When communism has went up to the point where you get the "Communism on the Rise" in your decisions tab, Press the Prepare civil war option when enough government points have been made. This will start a civil war. hopefully you have done this before the event for the Real Spanish civil war began! defeating them should be very easy to accomplish.

If you have an issue with this guide, or need me to edit to specify some parts of it, please tell me!