Open up a photo editing program. In order to successfully create posters, you should use an advanced photo editing program like Adobe Photoshop. This will allow professional layer edits that can make your poster look like the real thing.
Download the official movie poster that you want to spoof. There are several movie poster websites available, but for quality images, you should visit IMDB.com or IMP Awards (see Resources below). Save the poster image files to a folder that you can easily remember.
Write down your ideas for the movie poster spoof. It can be a simple change of the title, a remake of a trademark image or a completely different theme in general. Keeping your thoughts organized makes it a lot easier during the editing process.
Break the real poster into different layers. Separate all of the different images and titles, so they can be maneuvered easily or moved around. Label each layer of the poster, so you stay organized throughout the project.
Manipulate the title to something that catches the eye immediately. For example, "Star Wars" has been spoofed into "Thumb Wars", "Store Wars", "Spaceballs" and several other titles that blatantly copy the name. Think cleverly, and come up with something original and funny.
Stick with the same font when creating the new title. This will lend legitimacy to the spoof poster and add more to the comedy. Most of the time you can find the same font from a website like 1001 Free Fonts (see Resources below).
Replace the actors or characters in the poster with a new scenario that will create laughs. For example, instead of a bus on a "Speed" poster, you can create a poster with a picture of a playground that says "Speed 6: Swing Control". The more ridiculous the spoof, the more clever the poster will be.