This ties in to the achievements (Unfortunate Accident, Reconquista, Sibling Rivalry)
Start as King Alfonso VI of Leon in 1066. He has a claim on his younger brother Garcia II of Galicia's kingdom. Ideally declare war for this claim after Sancho has been dealt with and you have inherited Castile (See below) as you will have more troops and will be in a better position. Additionally, marry Adelaide of Ponthieu before unpausing the game at the start, this will give you an alliance with William the Conquerer who hopefully will be able to assist you in this war.
DO NOT Declare war on Sancho II of Castile if you want the Unfortunate Accident achievement in this save, as taking his kingdom via warfare will invalidate that method. I have done a guide for what to do regarding Sancho on the Unfortunate Accident achievement. Additionally as I have already mentioned, this will make your life in the war easier, if you do wish to tie this in to 1 achievement run with the previously mentioned achievements.