Select King Alfonso VI Campaign (1066, Iberia in Pieces)
On the bottom of the screen you will see "game rules" click on this and enable Ironman. Ensure achievements are obtainable. Start the campaign.
Open the Decision Panel (F8) and note one of the major decisions is to "Create Cadet Branch".
Clicking on this will show the requirements, of which all will be met except one, not being third in line or closer to the Dynasty head. At the start of the game I was 3rd in line.
Let the game run (on maximum speed) for a while (within a few in game years I believe) until the requirement above is met. One of the options in the task list at the top of the screen informed me I could Create a Cadet Branch as soon as it became available. There is no cost, just need to wait to be bumped down the line of succession.
I'm not 100% sure on how all the mechanics work, but I assume its because the current heir in front of you starts producing offspring which take precedence in the line of succession.