Begin by brainstorming ideas. Jot down all the things you love about aphmau's work and how you could expand on them. Consider the relationships between characters. Are there any that you think could be explored more? Once you have some solid ideas, start writing. Don't worry too much about making it perfect at first. Just let your creativity flow and keep in mind the essence of aphmau's world while adding your own unique touch.
First, you need to be really familiar with aphmau's work. Watch all the relevant videos or read any related materials. This will give you a good understanding of the characters, the setting, and the overall tone. Then, think about a unique concept. It could be a 'what - if' scenario, like what if a certain character had a different power? After that, start creating an outline for your story, including the beginning, middle, and end.
Well, start with your favorite part of aphmau's world. Maybe it's a particular character or a specific location. Build your story around that. Don't be afraid to put your own spin on things. For example, if you love a certain character but think they could be more complex, develop them further in your fanfic. Also, read other aphmau fanfics to get some inspiration on how to structure your story and what kind of language to use.