The claim throne scheme is unlocked via the meritocracy perk (stewardship tree - administrator focus).
A good strategy for getting this easily is via Duke Nuno Menez of Portucale in the 1066 start. Portucale is within Galicia in western Iberia, so you will need to zoom in to see it in the ruler select screen.
Firstly, unlock meritocracy if you did not begin with it (it is the first unlock in the focus so won't take long). Then select "claim throne" under hostile schemes on your liege - King Garcia Fernandez of Galicia. (click on Galicia, then right click on the King's picture). The scheme will take a couple of years, and there is a degree of RNG in whether it succeeds, but the odds should be in your favour.
The whole process takes 3-4ish game years. If you do this at 5x speed, it can be done in 10 minutes. If you get unlucky and fail then just restart. There should be an over 50% chance of success, so it shouldn't take many attempts.