This requires that you build a spy network up to 60+ have enough gold in the treasury for the support cost. which depends rebel size. roughly 100 gold or so. this will raise the chance for rebels by 10% so to save time do it towards rebels that has already been spawned. You now needs to joining them in the war using the support rebels casus belli. Which require that you do this and enforce their demands. When the rebel has occupied 50% of the country long enough it will break and the achievement 'Viva la Revolución!' will be yours. if this happens the rebels will disappear and their occupied provinces will return back to the country your at war with. When you have 50%+ of war score you have enough to demand this in a peace treaty. But go ahead and earn some more so you get something more for the trouble.