First, think of a situation that would make Ash cry. It could be the loss of a Pokemon, a broken friendship, or a failure in achieving a big goal. For example, if you want to write about the loss of a Pokemon, describe the bond Ash had with that Pokemon in detail before the loss. Then, show his immediate reaction, like his eyes welling up with tears.
To write an 'Ash crying' Pokemon fanfic, you need to understand Ash's character well. He's brave but also has a soft heart. You can start by creating a conflict. Maybe Ash is tricked by a false ally. As the story unfolds, build up the tension and emotion. When it comes to the crying part, use descriptive language. For instance, 'Ash's shoulders shook as sobs wracked his body, his tears falling onto the grass where his Pokemon once stood.' Add details about his surroundings and how his crying affects those around him.
Begin by choosing a theme for the fanfic that would lead to Ash crying. It could be related to his dreams being shattered, like not being able to participate in a major Pokemon tournament. Develop the plot leading up to this moment. Make sure to include his friends' reactions and how they try to comfort him. For the crying scene, think about what Ash would be thinking at that moment. He might be blaming himself or feeling hopeless. Write his internal monologue along with the physical act of crying to make it a really engaging fanfic.