bash them they know who boss
get some mercenary keep them close (army) otherwise they bash everything and you will be poor soon.
war is the best way to cash up. tons of gear to sell.
i dont chop heads this time just prisoneer them, works well with patch e1.6.x.
if you fight a enemy and you let him go you get relation improvement. in peacetime you can barter with people to get relation even right before you try to convince them to join you.
on map press K go to diplomacy check out who is the weakest kingdom (it doesn't matter if empire or not), klick on the leader portrait then on the clan portrait then again clan portrait left side.
klick through every clan of the kingdom find one without fief klick the leader then klick the star left top, add to bookmark. repeat this you can bookmark a lot of people.
on map press N go to heroes klick the first one, now you can switch them through with the next/previous page button at the right bottom corner so you can check quick if they moved.
klick right top last seen near location and activate the track button.
my 13 clan's needed between 1 and 400.000 denar each to convince them. one guy had a relation of -22 and i got him. save the game before you start the try sometimes it takes a couple of shots to get them. i save the game before leaving my kingdom if enything go wrong i reload after i found a clan who join me. reload clear problems of the kingdom and heading toward the one or more i know i can get.
but yeah the origin is real hard especially if more kingdoms declare war on you and you have to make peace and pay them. if you fight two kingdoms make peace with the stronger one. fight the other until you receive cash or at least that you not pay to much. save and try to convince weakest of them. declare war to the other kingdom fight a few troops to drop the payment. they can have more total strength but if you kill enough troops they will pay you so avoid the big armies and kill the little one's.