This quest is initiated after speaking with Nightingale during the main quest Keeping the Peace after you tell him that Theresa saved you from the bandits at Skalitz.
Speak with Theresa and ask her how she has been. Be sure to speak with her before noon so that she is able to go for a walk with you before beginning her daily duties.
Note: You can purchase a gift for Theresa in the form of jewellery, a dress or some other appropriate item (even a dagger!). This does not contribute to the outcome of the quest but functions as an aside to the story.
When Theresa asks where you should go for a walk, the only viable option is 'A stroll along the river'. You can select 'The tavern' and 'Gallows Hill' for the extended dialogue, but they are inappropriate for this time.
Begin the walk with Theresa along the river. When Theresa starts running, run ahead to ensure that you do not disgrace yourself, but not too far (Theresa will stop at some point to sit by the river). Once she arrives, sit next to her and exhaust the dialogue options.
You must now wait 2 - 4 days (or until Henry says 'I wonder what Theresa is doing now. I should go and see her again') before you can follow up from the walk. Be sure to do so after 19:00 this time.
Having won the fight, the objective is completed. You must now wait another few days until the objective updates before you can see Theresa again.
The next objective will be to play Blind Man’s Bluff with Theresa. Follow her to the barn and wait for the game to begin. It is entirely up to you whether you let her win or not.
After collecting the laundry, the final cut scene will initiate in which Henry and Theresa complete their courtship in a steamy sex scene.