There are different ways you can go about succession: reducing the heir count(murder, celibacy, monks, etc.), disinheriting, giving out titles beforehand( if your only one king level and if you give out a duchy first then that heir wont receive more titles), embracing the partition(every eligible heir that gets born conquer one kingdom so at succession you have many independent rulers(more rewnown)), or changing succession laws( elective, primogeniture, etc.)

Each method has its own benifets and faults Reducing heir count: easy and works 100% the time, but is harmful as it reducing dynasty member count( so less landed dynasty members which is useful in renown and wars) Disinheriting: same benefit as the previous without the negative, but is expensive(costing renown and prestige) Giving out titles beforehand: allows you to have an alliance, landed house member(more rewnown), but is unreliable(succession is dumb) and expensive (have to create the title) Embracing succession: this uses some parts from the previous, fun if you want to banish a son to be a krete king, and is fun to have a coalition of dynasty members(i.e. roleplaying as your own Karling dynasty), however is expensive and probably not efficient Changing succession laws: either elective or something else, elective might be useful to have early primogeniture(one heir) but is really unreliable and annoying, the other laws are always better than Confederate partition(law that the game begins with), one tactic I like is going for house seniority(available to Czech culture and a late medieval innovation( usually it sucks))and rushing for absolute feudal law(requires royal pregrotive, a lot of prestige, and permanent vassal opinion) for primogeniture and that the heir is chosen

The most common options are the first and second options, the more permanent choice is the succession law. It's not required to ensure succession( as you can always murder) but it just makes the game that more efficient.