Recruit Astarion early and keep him in the party often
Be consistently supportive, witty, and flirtatious in dialogue
Approve of his autonomy, freedom, and self-interest
Let him feed on you when he asks
Side with him in conversations about his past and boundaries
Avoid harshly judging his vampirism or his manipulative behavior
Help him with the vampire spawn situation and his personal quest
Encourage him to choose what he wants for himself
Protect him from being controlled by others
Trigger and choose romance dialogue options at camp
Long rest frequently to advance companion scenes
Keep his approval high
In Act 1, flirt with him and support his choices
In Act 2, continue backing him during his personal storyline
In Act 3, help him confront Cazador
Choose the romance resolution that fits your desired outcome for him
Stay loyal if you want to keep the romance active
Do not push him into choices he clearly rejects