If the Fate of Iberia DLC is installed, disable it to avoid the requirement of ending the Iberian Struggle for creating the Empire of Hispania. Begin as Alfonso, who has high Intrigue; with some planning and a bit of luck, you can eliminate your two brothers within the first two years to inherit their kingdoms (it might take a few months for agents to be willing to join your murder plots). Next, press your claim on the Kingdom of Navarra. Once secured, use the Unite the Spanish Thrones decision to consolidate León, Galicia, Castile, and Navarra. This provides a strong foundation to expand throughout the Iberian Peninsula until you're ready to form the Empire of Hispania. You do not need to complete this achievement within a single character's lifetime.