To write a good fanfiction about their kissing scene, you need to be familiar with their relationship arc throughout the Harry Potter series. Think about the moments that led up to this. You could have Hermione make the first move, which would be a bit out of character but interesting. Describe how Ron reacts, his eyes widening in surprise at first and then melting into the kiss. Also, don't forget to add the little details like the warmth of their bodies close together and the sound of their breathing.