1. Start
I recommend starting as a
small nation this makes it easier because you do not have to overlook that many provinces etc. AND YOU DONT HAVE TO PAY THAT MUCH TO INCREASE YOUR KINGDOM POWER.
Maybe you also should choose a country that is near the edge of the map but that is not neccessary.


2. Taxation
Dont collect taxes from your people it only will cause them to rebel and start and uprisings against you.
Make sure your income only comes from trading and vassals. For the start use your royal family members as merchants. You need atleast 4 merchants at your royal court. Make trading contracts with big nations like byzantium and germany....


3.Investing your gold
First invest your money in KINGDOM POWER UNTIL IT IS MAXED OUT. Thats why it is important to start as nation with a single province. Then build up and army and annex and vassalize nations. Give your money to your vassals its like investing in stocks you help your minors to build up their nation while at the same time you also increase the relationships with them and you will PROFIT more from them. Make a pact of non agression with all your vassals so they dont attack you.

4. Warfare and vassals
ANY CASTLE CAN BE EASILY TAKEN YOU JUST NEED 2 MARSHALLS WITH A COMPLETE ARMY AND LEADERSHIP AND FEAR TRAIT ATTACK THE CASTLE MAKE SURE YOU ATTACK AND NOT SIEGE THE CASTLE.
Vassals can be pretty useful because you can call them in anywar and they have to follow you.
This has to be done via the DEMANDS section while you have an audience with them dont make an alliance with your vassals this will only turn everything the other way they will call you in their wars.

5.Short vassal guide
Some nations cant be vassalized because they are already have an overlord. Some wont accept your offer because the relations are low. Release some prisoners you have captured during the war. Make sure your marshals have the FEAR and LEADERSHIP trait. Max them out thats good when you have 2 armies because each marshal can level one each. With this marshal combo nobody can beat you on the battlefield. You will capture alot of enemy marshals as long as you have a free place in your court.

6.Endgame
Repeat all of the above you will get bigger and more powerful after a while. Start vassalizing major nations they will vote for you in the elections.

TIPS FOR YOUR ARMY MARSHALLS
USE LEADERSHIP AND FEAR AS SKILLS TO LOWER YOUR ENEMYS MORALE THE ENEMY MARSHAL IS MORE LIKELY TO SURRENDER THEN. If you capture enough marshals of the enemy nation maybe even the king the nation you are at war with is most likely more willingly to become your vassal and to accept your peace terms.

Thats it.