Purchase and install Photoshop or a similar software program on your computer. Ensure that the program performs all the tasks necessary for making a poster (i.e., importing photos and artwork, cropping photos, adding text boxes) and open the software to create the poster.
Determine what you want the selling point of your movie to be. Is it a monster or crazed killer in a mask? Axe, knife or other weapon? Most people will only take a quick glimpse at the poster, so give them an idea of what they'll be in for, but remember to use subtlety; showing too much can eliminate the element of surprise and dampen curiosity.
Use jarring, high-contrast colors for the color scheme of the poster. Colors like red, orange or yellow against a black or blue background can create attract the viewer's eye and create a sense of unease. However, keep any gore to a minimum so as not to limit the number and types of places the poster can be displayed.
Have some or all the elements of the poster, including text layout, font and illustrations/photos be somewhat disproportionate and off-kilter compared to standard movie posters to further hint at the scare-fest that awaits within.
Create a tag line for the movie--something short and snappy that hooks the viewer and sells the concept of your film. Use creepy, non-standard fonts and font sizes for the tag line, title and any names/credits in the main body of the poster. And, set some of the text at an angle if you choose, to heighten the overall effect.
Insert text boxes for any positive quotes from critics or other sources, in a genuine or even ironic sense, to promote the quality of the movie as a whole, or any specific elements within the film. Also, decide whether you want to place any or all of the standard credits (producer, writer, director, etc.) at the base of the poster.
Save your project when finished and transfer a copy of the file to a flash drive or other removable media. Take it to a facility that provides poster printing of various sizes, such as FedEx Kinko's, to have your poster printed for display.