This question has been asked and answered many times. I would recommend the search function.
But gladly, one more time:
1. Build a relationship with Claire, increase A. Follow the storyline with her until you have had sex at least once.
2. In the evening, go to her kitchen and peel potatoes for her. Talk to her again as soon as you have completed this daily task.
3. She uses an excuse to lure you down to the basement where you secretly have sex.
From here it continues.