Use simple, spooky elements. For example, 'Dark room. Strange noise. Gone.' Just pick a common, yet eerie setting like a dark room, add an unexplained and menacing sound, and then a final word that implies something bad has happened.
One such magazine is 'Flash Fiction Online'. It often features very short stories, including six - word horror ones. They like to showcase different forms of short - form writing and have a diverse range of horror content within their pages.
In my opinion, a great 'horror six word story' can convey a lot with very little. For instance, 'Old photo. Eyes. Follow me'. Just these six words can make you feel uneasy. It could also involve a sudden twist, like 'Happy face. Mask. True horror'. The contrast between the happy face and the underlying horror is what makes it work. By using simple yet powerful words, a 'horror six word story' can send shivers down your spine.
I'm not sure specifically which '20 best six - word stories' you refer to. There are many collections out there. Some might be about love like 'Met you. Loved you. Lost you.' Another could be a sad one 'Birthday. Alone. No calls. No cake.' And there could be a hopeful one such as 'Struggled. Fell. Got up. Kept going.'