Progress the main story until Katherine becomes available as a romance option
Choose supportive, respectful dialogue options when speaking with Katherine
Complete any quests involving Katherine and keep her favor high
Avoid rude, aggressive, or overly flirtatious choices that can hurt the relationship
Help Katherine during her related questline and side objectives
Select the romance-specific dialogue prompt when it appears
Finish the required story events that unlock the romance scene
Keep your reputation and relationship status positive in the relevant interactions