Hi lili hong,

This is a very generalised question to answer. You can refer to few of the courses available on Mendix Academy which explains step by step to build a microflow.

https://academy.mendix.com/link/modules/92/lectures/665/5.1-Adding-Custom-Logic 

https://academy.mendix.com/link/modules/324/lectures/2459/7.1-Introduction 

Incase is there is a specific use case where you are struck and need help, you can share that and someone will try to answer to it. :)

I  created that microflow, yes.