Play the Peninsular campaign in order to limit the amount of enemies that must be managed and to already have a favorable number of regions under your control with a garrison of soldiers in each city. Additionally, select the easy difficuly setting. Progress through the campaign as normal. Keep a garrison in each city and lower the taxes to the maximum amount or cut it altogether. After several turns, some regions will begin to be Pro-French. Focus on those areas one while getting profits from the others.