To write this type of fanfiction, start with a strong concept. Consider how the world of Equestria would look like from Twilight's perspective as a blind pony. You can describe the sounds, smells, and textures in detail. Also, think about the challenges she would face in her daily activities like going to the library or attending Celestia's lessons. Build the story around how she overcomes these challenges, whether it's through her own determination, her friends' help, or a combination of both.
First, you need to understand Twilight Sparkle's character really well. Think about how her normal traits like her love for knowledge and her friendship with the others would be affected by blindness. For example, she might be more frustrated at first when she can't read her books as easily. Then, come up with a plot. It could be something like she has to solve a mystery in Ponyville without relying on her sight.
When writing a 'Blind Twilight Sparkle' fanfiction, it's important to do research on how blind individuals adapt in real life. This can give you ideas on how Twilight would use her other senses. You could also add some magical elements to it. For instance, maybe she discovers a new spell that helps her 'see' in a different way. And don't forget to develop the relationships with her friends. They would play a crucial role in her journey, like helping her get around or providing emotional support.