... Hmm, I can't give you any art tips like composition perspective color etc so I will just give you my opinion as horror fans : make a story with normal daily activity then twist it.
For example this short movie have a setting story of a woman found a hair on her kitchen sink then she decided to clean it. Turn out... Watch it yourself I won't spoil it. But it do makes you think of what will happen if you pull a hair on your sink lol. Link : https://www.youtube.com/watch?v=lt58gDgxy9Q
Or the Lights out, the short movie not the one that 2 hours. It's basically just a simple idea, you turn off the lamp and bam, suddenly something unusual happen.
Or, you can just make something that totally not normal happening from beginning. Like that popular story : 2AM: The Smiling Man. There is a short movie and a short animation for that story, you can decided what's scarier for you ( https://www.youtube.com/watch?v=_u6Tt3PqIfQ ) and ( https://www.youtube.com/watch?v=hD5hYfZmdwY )
On Junji Ito topic, I found this interview that well, I know you say no JI, but if you read, he say that his works is influenced by many things too (Gyo from the Jaws etc) so well, my advice is take things that interest you and twist it to your own taste.
Sorry, can't give any art advice as I say so well yeah, tl;dr just get a daily life activity and then make it supernatural lol.
Hope it helps [or maybe not? ._. ]
Edit : lol forget the JI link : http://www.78magazine.com/issues/03-01/arts/junji.shtml