Firstly, during a time of peace I buy several enterprises throughout the lands so that i can easily fund my army even without fief support for a while. I farm relation as much as I can with neighbours of the castle or town i target for the capital. (I.E Farm relations with Nords, Swadia if i target Curaw or Reyadin) I train my companions (warrior ones like firentis and alayen, not jeremus and deshavi) With Charisma, leadership and training, as well as the highest, noble like equipment money can buy. (I like heradlic mail and huscarl shields, and chargers if I can find em.) Before I go to siege the town, I recruit several mid tier troops to fill out the garison when its conquered. (Nord Footman, Nord archers, good reliable mid tier, unexpensive troops but still useful in a siege

After I conquer the castle, I play defensively, prefering to keep my small amounts of fiefs and castles maintained than having large amounts of looted or attacked fiefs. And from there I slowly take over the map. (because im running diplomacy, I'll often sign non aggression treaties with the strongest of my neighbours, so there is no risk of a two front war I cant win.) And thats pretty much it.