You marry your male heir to the daughter of a King.
When/before the King dies, murder his son, the heir/king.
Murder the eldest son's children if he has any.
Murder the next son, his kids, and so on.
Until it comes to just the kings daughters.
Preferably your heir married the eldest daughter, if not, you'll have to murder her and her kids.
Keep murdering those girls until your daughter-in-law is the heir. Then murder the king.
The children of the union between your heir and her will produce a child that will inherit everything.
This is rather labour intensive and risky as being a "Known Murder" is a curse that never lifts.
There are other options - for example:
Firstly, marry the youngest daughter - so that she's most likely to live longest after her father dies.
Wait for the father to die.
OK - so his son probably inherited.
Start a plot against the son but keep it with low-ish plot power so it shouldn't fire.
As you're plotting to kill him there's a good chance he'll go into hiding.
Declare war when he goes into hiding in favour of your wife.
Install your wife.
If the new king is a minor or actually a Queen you can just declare war to press your wife's claim, you can also declare war if there's a succession crisis. You might achieve this by spying on him and starting vicious rumous. If you are able to kidnap and imprison the King you can also press a claim and you will auto-win due to him being imprisoned. You can also wait for him to go on pilgrimage or become incapable.
Essentially, anything that generates either a war for the title or a regency will allow you to press your wife's claim.