This trophy will be unlocked after you have seen the third-note event for someone. Now, the only thing I'm unsure of is if it has to be a bachelor/ette or just anyone in general. I am trying for Dean in my playthrough and, once I finished the cutscene for his third note, the trophy unlocked for me. He is also the highest in affection on my playthrough, so perhaps if there are scenes associated with other characters, this might be an option. That being said, I have not had any other scenes make themselves apparent outside of bachelor/ettes, so do take that in mind.
Seashells and their commonly preferred Love items are the best way to obtain heart points with the villager, though you can also gain some points by fulfilling their requests. If you need to find where the villager is currently, be sure to use the touch pad and bring up the map, where it will show you their general locations. Characters may fade in and out with the change of the hours, so if you get there too late and find that the character is gone, check for their newer location. This isn't like previous games where you could stop them in transition.