Make sure to appoint companions that are nobles with either good natured or upstanding personalities (in particular Sir Roland) this will make it immensely easier on yourself.
You should make some companions into lords of the villages attached to your walled fief ( castle/ city) so that they can start gathering an army as soon as possible. Your next step is to hire a Mercenary Company to support you in defending your new kingdom, mainly against the kingdom you took it from, as they will attempt to reconquer it.
