Use cav heavy armies and higher militia as you get them is the main point, second being to sack every settlement you come across.
In your cities, build the barracks line of buildings with at least 1 brothel to train spies to open gates for you. Train spear militia and higher.
In castles, train mailed knights (christian faction) and use those to move around quickly and devastate enemy armies quickly using their charge.

Early on, you need to use all your generals and garrisons (just keep enough so the settlement does not riot) to besiege any nearby rebel settlement and take them swiftly before the AI can.
Concurrently, use your diplomat/princess to buy settlements off the AI for alliance/map/trade or florins.
Any new settlement you take is to build more troops immediately when possible and continue expanding.

Fast-paced game, slightly boring as all battles will be Charge-Win or extended melee-Win for sieges. Don't bother with ranged troops early on. Just heavy cav and loads of cheap infantry.