Well, unfortunately you cant do that. It comes as like a "package" and u have to rate them all togethehr, BUT there are some multiple story mangas that get oneshot mangas and put it in there, so try searching up each mangas story seperately cuz they may or may not be a seperate oneshot
Well, I usually just give an overall bad rating (for the meh stories), then consider the good story and add a star. So if for example I rated a story 2 stars, I give it 3 stars.
You can write a comment that you can see when you rate the manga - then highlight the chapters you liked, and write what you thought of the others ... see this image for reference http://www.mangago.me/home/photo/2214993/
Hey I'm just wondering how do you guys rate a multiple stories manga, where you only like one of the stories and the rest are meh?