I like how Yamada's relationship development is pretty natural but Ichikawa does a complete 180 after volume 1.
Ah, in any case, Ichikawa's conscious shift to "I like Yamada" is definitely chapter 14. As for Yamada, the shift to consciously thinking Ichikawa is funny might be the bike chapter in chapter 6, though it could have been sooner. Yamada thinking Ichikawa is interesting likely correlates with chapter 15, presumably because she was aware that Ichikawa was the one who gave her the tissue (this is also a likely point where Yamada started unconsciously liking Ichikawa, much like how Ichikawa unconsciously liked Yamada since the start of the manga). As for her consciously liking Ichikawa, it's definitely the milk tea incident or if not that, then soon after in chapter 30.
One thing I would like to add is that they were both likely somewhat conscious of each other's feelings by the Christmas date, though I'm not sure if Yamada got an inkling of Ichikawa's feelings earlier than that.