This requires that you build a spy network up to 60+ in a country, 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 can increase the odds that they win by joining them in the war using the support rebels casus belli. This will also line you up to get the "My True Friend" achievement. 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 this achievement will be yours. for "My True Friend" you can enforce it by reaching 50%+ war score and demand that they fulfill the rebels demands.