To add a bit of specificity as to process - Each winter, militia units and Native American war parties go home. In the spring the war parties reappear. New militia are raised twice a year, soon after the new year and June (I believe). These militia are built in proportion to the number of cities you control in each of the four grand regions, New England, Middle States, South and Deep South.
Because the units arsan refers to in the ledger cost EP, (EP are the money, WS and conscripts all rolled into one) you need to maximize how much EP you generate. Using privateers is one method. In general, put at least one in each of the four sea zones. This gains one EP for each sea zone in which you have a raiding ship.
Second arsan's statement about the game, my top game to play.