To use the thief, select it and click on the enemy's treasury. The thief will automatically move there and sit there, but your gold stocks will rise slowly.
Beware that the thief's true identity can be revealed by the enemy clicking on him or nearby guards. The thief will also reveal himself when he engages any enemy unit or peasant.
Haven't played S2 in a looooooooong time (I thought it was lord of the realms lol). Seems like a complex game.
SH2 is not that hard once you understand the new things it introduces.
The honour system is what greatly expands the list of buildings, products and so on. If you have played Legends, this thing shouldn't be unknown for you. There are just so many ways to acquire and use honour :D
The other thing is the negative factors you must contain so that they don't wreck your popularity. These are: