Open the movie in which you wish to add a subliminal message in an editing program such as "Final Cut Pro" or "Adobe Premiere."
Open the program's "File" menu and choose "Import." Navigate to the image or footage that you wish to splice subliminally into your film. If you're using written text, use a program such as "Paint" or "Adobe Photoshop" to create a still graphic containing the text in your desired font, size and color, then save this graphic and open it in your film editing program.
Open your subliminal video or graphic in the program's editing window and set an in-point at the place where your desired subliminal image is displayed. If you're using a still image, the in-point can be any point on the time-line.
Skip forward over one frame then set the out-point at the next frame. Your subliminal image should now have a run-time of two frames, which is displayed as "00:00:00:02" in an hour-minute-second-frame time-line readout. Any longer and viewers will recognize the image; any shorter and it will be lost in compression.
Drag your subliminal message onto the point on the timeline where you want it to appear. Subliminal images placed in the cut between two other images have a subtly perturbing effect; placing a subliminal image in the middle of a shot, breaking the flow, is more jarring but also easier to spot, especially on a smaller screen.
Open the "File" menu and select "Export" to output your movie with the subtle message.