To make the shift, start by analyzing the elements of fine art and trying to incorporate them into your cartooning. Experiment with different mediums like paints or charcoals. Also, take classes or look at tutorials to learn the specific skills needed for fine art.
One of the main features of horror one shot manga is the ability to build up tension quickly. They usually have a limited number of pages to convey a terrifying story, so the pacing is crucial. Also, the art style often plays a big role in creating a spooky mood.