Unless you have a high reputation with any of the Boyar's, you'll be hard pressed to recruit any of them without any territory. A few can be easily turned if you appeal to their greed (promise land) or their honor (it's the right thing), but the majority will not. Pay attention to what they say and choose the response to convert them that best fits their personality. You'll see it enough that you'll eventually figure out what type of person the lord is after a few interactions with them. But beware, you only get the chance to convert a lord through dialogue -once-. If it fails, he will not let you try again.
I would start by taking a piece of land that's far and away from their main cities - something on the outskirts of their territory and would take time for them to get there. You could try capturing something on the outskirts near a faction that's at war with them as well, since it's possible that they will be around the area to help defend your new land (but, alternatively, your enemy might also be there attacking that faction). Once you have your starting point, build out from there to keep everything you need to defend close together and easier to get to. At that point, you may want to start recruiting lords, but only those that are close by to your territory, as they'll be easier to convert (they will often say "you are in no position to help me if I'm in trouble", which means "you have no land nearby").
Claimant quests can be tough, but keep at it. Sometimes you'll think victory is impossible, and other times you'll work your butt off and win a few key victories. Just remember you're outnumbered, so you need to play it safe, but aggressive. Get rid of their armies by luring them away from help and taking them out 1 by 1. Don't be afraid to take some of them hostage from time to time too if you need the extra money, or to keep them from coming back with a new army so quickly. Alternatively, don't be afraid to let them go either if you have the option. Once you finish the quest, these lords will be part of your kingdom, so try not to piss them off to much - otherwise you'll see a lot of them leaving for other factions and that can be bad for you later.