The best way is to be a vassel for a while and level up till you can carry about 120 troops. While your at it make about 4+ friends in the faction your vassel to with relations of 25-40, the higher the better obviously, who all like each other or at least dont mind . This means when you start your own kingdom you should be able to recruit at least 1 of those friends. The hardest part is starting the ball rolling because you possess only a castle and they wont want to join.
How i started my faction which is now booming:
Had 5 friends ranging from 25-50 relations in swaddy lands who were mainly from the same family or my marryed family.
Took a castle in the south of the rhodok territory right next to their city so i wasn't going to be at war with the world and only rhodoks. I marched my 200 men (i cheated because i already owned 2 castles and a city and thought it a waste after recruiting all those units so i upped the leadership bonus to carry 200 units then put it back down when i transported my units to my new castle) down to that castle and took it, gave one of my companions the village the castle came with and marched on to the city right next to it once my companion built a small army.
I then made a mistake, i gave the city to my companinion before garisoning it. The ai will build a garison for it but it takes a long long time, which is fine if you have a small kingdom but i was at war vs rhodoks entire army and gave the city to my companion who took ageees to garison and has only just managed to get 300 units in it about a week ago and my kingdom now has like 10 lords in it with 2 castles, 4 citys.
Instead i should of chose to keep the city for my self and then put at least 100 men in and given it to a lord as a pressent for joining me or maybe my companion. Then it wouldn't of been defenceless so take that as a note.
After this i desided to recruite lords. SAVE before you try each time becuase you get 1 shot, if they reject you you cant recruite them again. This is obviously very bad if they had 30 relations with you. Because all my friends in swaddy land liked each other because they were family once i recruited one they became easyer and easyer to get and when i recruited trhemn they brought their land too. They keep the castle/city but the towns are yours to dish out. I usualy give them back. I got suno and uxvil or what ever it called from just getting the lords to join. Then i took yael and a few other places.
Make sure you keep the marshell circling round, it will keep controversy down. Also you can gift the marshel and take it away and gift it again for small relationship boosts. Im going to do this to one of my vassels who i dont want to give much land to and every time is loosing relations with me. Its silly thing in the game, i now have 10 lords. I give land away they all take a -3 hit and one person gets a +10 hit. This means gradualy they all hate me because they loose more relation then they gain.
However, there is a way round this. if you ask them who they support and you gift it to the most supported person then they dont complain or ***** and get + relation points if you give it to who they supported.
However the game is also bugged because the only tyhing i can ask is who they think shouldget norma, which i gave away ages ago. But it seems to work anyway even though im gifting diferent feifs.
Hope that helped anyway.
