Catch and Release is your friend.
Seriously, it is about the fastest way to build relationships with enemy lords. Don't siege. Don't Raid. Just catch and release as many Lords as you can. Once you get the relationship up very high, not only are they easier to recruit but they are much much cheaper.
For example, I just recruited my Wife's Father's Clan into my Kingdom the marriage related relationship gain and using catch and release, I actually managed to get 100 relation with him. I then started a war with his faction and went to recruit him. The persuasion was easy and he only asked for 15k in barter to join. He was a Tier 5 Clan and brought with him, 2 Towns and a Castle.
One thing I noticed that most people seem to jump into the whole player Kingdom things way too fast without the proper preparation and struggle because of it though honestly it is probably due to the way the story quest almost forcefully propels you into creating a Kingdom. Ideally you should wait a good long time, as much as possible before creating a Kingdom so you can do your prep work.
Keep in mind that I generally totally ignore the quest so what I have no idea if what I am suggesting will work if your doing the quests or not but it should. Also this is the "No Exploiting Smithing" way.
1. Keep your army costs minimal. Just get the minimum number troops and keep then down tiered as much as possible.
2. Focus on getting as many workshops up an running as possible and while it might cost a bit of money, make sure to experiment until you find the product with the best return. Tanneries for me always seem to have high returns, better than most anyway and it does seem like the location matters with Tanneries. Make sure to buy new workshops whenever you can as your Clan Tier levels. Also once you get those workshops up, start getting caravans out there ASAP as well.
3. Once you have your 3-4 workshops up and running, have 2-3 caravans out and about and if you have kept your army size and tier to the bare minimums you can get away with, you should have a decent passive income by this time. Save as much as you can.
4. Once you have a nice nest egg of money, start building up your army in terms of numbers and quality but ENSURE you are still making at least a fair amount of passive income for your workshops so that the wealth keeps building.
5. Join a faction as a Merc any faction, hell keep jumping between them all if you want and execute operation "Catch and Release". Basically do everything in your power to catch every enemy lord you can and release them every time. This build not only relationship with these lords but also your Charm skill. Also absolutely avoid initiating a siege because it is like a -30 relation hit with the owner of the fief however you can join an army or "Assist" with a Siege. Also don't raid because this is a negative relation hit well. You want only positive relation actions in this phase.
Note 1: Speaking of Charm and a few other skills, this, not weapons and such, is where you want to spend you skill points. You will need a high charm to make it easier to recruit lords. Also I think Leadership, Cunning and a few other skills or attributes will come into play with convincing lords to join you so you want to be an "Intellectual" not a "Brute" if you want to do a player kingdom and you can get by just fine with relatively low combat skills.
Note 2: Just becoming a merc should allow you to do the story quests without screwing something up so if your doing the story questions just stop here and keep doing the whole catch and release over and over as long as you can. Also, Save, Save, Save and Save some more. Save all you can because money makes the world go around. Ideally you want multiple Clan with over 60 relation with you. If you don't care about the quests you should become a vassal before creating your own kingdom for multiple reasons I will describe a bit later. Finally I am not sure if becoming a vassal will screw up the question. I kind of doubt it but I am not sure so do your own research if you want to become a vassal while doing the quests.
6. Join a Kingdom as a vassal and continue the "Catch and Release" plan. As a vassal you will be given a fief or most likely several fiefs all of which rapidly add to your passive income. Also continue to keep your costs as low as possible. For example use mostly Tier 1 and 2 troops for your garrisons. Also ideally if you plan on becoming a vassal on the way to becoming a king, plan ahead and put your workshops in the Kingdom you plan to join so you don't have to worry about losing them due to a declaration of war.
7. This is a big one with lots of instructions. Continue "Catch and Release" as a vassal. Try to convince your liege to give you more fiefs, preferably a town or two. Towns rake in the money which you will be needing later. Keep those town garrison costs low as well by using tons of Tier 1 and 2 troops. Also try for towns and castles that are clustered near each other as much as possible. This can be very important for creating your own Kingdom. As your renown goes up, you will be able to get more and more fiefs. My currently playthrough I had 2 cites and 4 castles before I started getting voted down for new fiefs. Always try to save several hundred influence to push fief decisions in your favor.. Also, you will want to start investing in as many companion led parties as you can reasonable afford and still maintain that passive income. You will want at least one but more are better. Once you have a companion run party, immediately create and army and take them into it. You do this because you want to work on your leadership skill. If you have listened to me you have invested heavily in the "intellectual" pursuits you should have quite a bit of attribute and focus points invested in Charm and Leadership so leadership should level up fast. If you afford most companion led parties and you take them to your army, leadership will just go up faster. Also while keeping companion led parties will lead to slowly losing cohesion it costs zero influence. All you have to do when cohesion breaks is call them back immediately and you will be back at 100 cohesion. You can basically keep these companion led parties around permanently so you can earn leadership experience permanently.
9. If you have been doing as I recommend, you will likely have a couple towns and few castles and most of the enemy lords you have been fighting will be huge fans of yours with 30-60+ relationship each. You will also have a large sum of denars saved up, maybe a couple of million or more depending on how much fighting you have did. Also you should have 3 companion parties aside from yourself to act as armies for your new Kingdom. Make sure all this is true and your good to go to the final step.
Note: You will have probably spent 800-1000 days in game by this point just to give you a sense of how much time this prep takes.
10. First, find a Lord in your current faction that you have good relations with, preferably the best relations with to be honest but you also want to consider what territories he owns because you are going to steal him and his fiefs for you new Kindgom. Once you have found that lord an he is not in a castle or in an army, go to your Clan screen and click leave Kingdom. It will ask you if you want to relinquish your fiefs and your going to say no. You will immediately go to war with your old faction. Once that is done, before anything else, create your new kingdom and then go back to the campaign map and engage with the lord you want to recruit. Tell him you need to discuss something and ask him about his liege. If you have did what I said and have decent charm, leadership and a few other skills, plus have a great relationship with this lord he should be easy to recruit. If you have 100 relation with him, he will join you with all his fiefs for like 15k denars. If the faction is lower it will cost more. Anything above 60 relation seems to be like in the 100k range tops. Boom, you have your own Kingdom, your first Clan with its 2-3 parties and all the fiefs he owned. While your you can go around doing this will all your old factions lords that you had good relationships with and take as much territory as you can if you want but I recommend just immediately finding a enemy lord and asking him for peace. Chances are the cost of peace will be zero and now you will have some time to recruit more lords to build up your faction.
Note: What I do at this point is go recruit at least 2-3 clans from other lords you have great relationships with. If you recruit them outside of war they don't come with their territories though so I usually try to grab the more distant lords I have good relations with and get them instead of lords I might want to recruit during war so I can get not only the clan and the territory. You will probably want to gift these lords with your castles once you get them so they have at least some territory but you don't have to. Make sure you keep your cities though because you will still need the passive income. Once you have 2-3 clans plus your own parties you should be able to fight a war without losing too much, especially if you immediately try to recruit a few of the "enemy" lords that regard you as "best friend". If all goes well you will add 2-3 new clans to your kingdom and all the territory they controlled without having to lift your sword once.
Anyway, that to me is the "Proper" way to create a Kingdom, at least if you don't want to struggle. Figure 600-1000 days to get here without mods, cheats or any other ez-mode strategies.