Find the image you want to use for the subliminal message. If your message is to try and get the person watching to drink something, look for an image of a beverage. If the message is to quit smoking, look for an image of something that will remind someone to quit smoking, such as a damaged lung.
Bring up the video in the editing program. If you are using an editing program, you may need to digitize the entire video in order to edit it on your computer. When the video is in the program, find the location where you want to place the subliminal message.
Edit the image down to a clip that is less than 30 milliseconds in length. This should be short enough that the viewer does not realize she is seeing the image, while long enough that the image theoretically registers in the viewer's subconscious.
Insert the clip into the video where you see fit. Because the image is onscreen for only 30 milliseconds, you can simply copy and paste over part of the video instead of inserting in between two sections of the video. Because it is copied and pasted over such a small section, the viewer will not sense an interruption in the video itself.