Some links that might be helpful. Given in the spirit of I have no clue how to do it. But I have often wondered what went into it.
https://www.er-central.com/authors/writing-a-graded-reader/writing-graded-readers-rob-waring/
https://www.youtube.com/watch?v=LWQ-YjR9Ipk
https://www.amazon.com/Graded-Readers-Training-Course-Writers-ebook/dp/B00F433CI0