In order to begin a takeover, the region must be destabilised. Completing the correct story mission will gradually destabilise each province until they are all red. When you are ready, speak to the man waiting at either the green or yellow explanation point on the map to start the takeover.
Completing a Guerrilla takeover of a village or a Roija takeover of a drug farm will net you 30xp with the appropriate faction. Completing a Guerrilla takeover of a town or military base will net you 50xp with the Guerrilla faction. To get the most xp from each province you want to do all the villages first and then do however many towns or military bases are needed to finish the province.
A very quick way to take over villages is to use the agency jeep. Keep as much distance as you can between you and the enemies so you wont destroy yourself. Equip the rockets in the jeep and let the auto lock on do all the work. Sit back and very slowly roll forwards firing rockets as fast as you can. When you get to the blockade, a couple of rockets should be enough to clear it for you.
For clearing towns, a helicopter works best. Just hover over the streets and fire at any military personal you can see to gradually raise the meter until you can land and swap the flags.
For military bases, its best to stay on foot. Avoid the roads as much as possible because the tanks don't think twice about firing their main gun. Just hide behind all the barracks and military buildings and destroy the blockades from a safe distance, hiding whenever a tank shows up.
Once you complete all the villages in the province, one or two towns or military bases should be enough to put it under Guerrilla control and stabilise the province.