So you need the 51% of the de jure counties as your vassals or in your demesne, and then create the kingdom/usurp the kingdom (if there is another king of your religion). If you want to become king of X, you need to become king of X. You can use the Subjugation CB once before declaring the ambition (which can be used to subjugate land outside the kingdom you are going to target, or even gain land that lets you declare an ambition to become king of another kingdom than the one you are currently in) and since it's such a powerful CB, it's worth using as much as you can get away with. If you are a pagan, the main use is to get access to unlimited use of the Subjugation CB, which makes becoming a king only a matter of being able to actually win the wars.

The use of the 'Become King' ambition when you aren't a pagan is merely to get a bit of extra presitge if you are a grand duke and are ready to become the king, anyway. If someone holds the kingdom title already, and it is their primary title, then you must conquer all the land they hold within that de jure kingdom. The normal way to gain the kingdom is to just conquer it piece by piece until you have 51% when there is nobody who currently holds the kingdom title, or it is not their primary title. You can get the throne by intrigue only if you are already a (distant) heir to the throne, and just murder your way up to being next in line, then murder the reigning monarch. You can't get a title by marriage within one generation, but you can marry the ruler and make sure the children are your dynasty, and inherit both kingdoms as your own child.

Originally posted by svyaz:How do I get it by marriage or intrigue?

Crusader kings 2 ambition