Step #1: Becoming Fascist
Okay so first you are gonna have to become fascist. This part is easy unless you either can't read or your game is broken. A few days in an event should pop up that either makes you Fascist or gives you a bit of communist popularity. Choose the fascist option.
Step #2: Civil War
Not one hundred percent sure about this part, there seems to be an element of luck in getting the civil war or not. As far as I'm aware most of the time you don't get a civil war if you're fascist, but I did, so I'll include it anyway. What you want to do is first kill their navy with your navy and bombers. Then have your navy surround the Iberian east coast. Put your fighters on intercept and air superiority over Iberian Region and close their pockets seperated from their main territory. If you are fast you can split their forces in two by driving a part that little 1 tile corridor. This should be easy as even AI on AI fascists win so with you at the helm it should be easy. Start justifying against Belgium IMMEDIATELY.
Step #3: Kill Belgium
Set up a naval invasion with your actual divisions even if the civil war isn't over. Navally invade Belgium (They have no navy so it should be easy). If at this point they are guaranteed then you are ♥♥♥♥ed and you need to restart. Draw troops away from the civil war once you land, you should beat them easy.
Step #4: Join Axis and kill France
Justify against France. Since you have taken Belgium you can just go around the Maginot line. Even if you contribute nothing to the war effort since the territory will go to you it will act like you took Paris with your own units. Easy peasy.