Talk to Theresa before noon on a sunny day and ask her on a walk. When you have the time, head back to the mill in Rattay. There are three more steps to complete to finish it. At the same time, you’ll get a notification that you’ve started the “Courtship” side quest.
When you’re on your feet again, you’ll eventually make your way to Rattay and take a job with the local bailiff as part of the “Keeping the Peace” quest.
She’ll nurse you back to health at her uncle’s mill in Rattay. She’ll return the favor later when you head back to Skalitz to (try to) bury your parents. A little later, you (can) save her from the attention of some soldiers as you sprint past on your way out of town during the “Run!” quest. Theresa’s part of your introduction to the game back in Skalitz - you have to give her some nails.
