Hello!!! If I'm not mistaken for this star you have to do the vanilla route, crawling the book and when shadow comes make him the proposal. From there everything normal as if you did the vanilla route. Obviously trying not to give points of affection to shadow. And that's it, if I'm not mistaken you won't be able to sleep and you'll find him.
Hope that it works for you ;3
Sorry if I had any mistake. I dont remember so well how I get that star.