Well, first is to know what your painting, and practicing the berry concepts that it's based off of. You can study specific styles and learn how other artists create in their style fairly easily. There is 4chan's /ic/ that might give a lot of general threads with different focuses on whatever style you might want to practice. or you can use e-hentai.org/tag/how+to for literal endless tips (it's not only porn on this site btw, a lot of guides on anatomy, proportions, rendering, you name it)
Usually, you can understand what a demographic might want from by being a counterexample of or developing upon established tropes. But this relies heavily on concepts that will be ever changing. Sometimes you have to take inspiration from what you enjoy and meld it together for your own artwork. and that usually takes a lot of time in developing an eye for what you want and time in experimenting with what styles you replicate and like!