Pick the movie you want to make a poster for and watch it a few times. Take notes on what images stand out for you that are a good center point for your poster. Do a search on the Internet for existing posters for your movie to get an idea of what other designers visualized.
Spend some time brainstorming what you want your poster to look like. Think about the poster's aesthetic, the message that you want to communicate and how to organize the poster so it's clear and not cluttered. Think about what important data needs to go on the poster besides the title, such as stars, director and producer, if they are noteworthy.
Download stills from the movie from the Internet when you have decided on your concept. Open Photoshop and open the stills in the program. You can also re-create an image or object that is key in the film and import the photo into Photoshop.
Resize the image in Photoshop, leaving room at the top and bottom for header and footer text. Play with Photoshop's Image options to get the picture how you want it. For example, you can blur the picture slightly or experiment with brightness and contrast. Erase any unnecessary background around your main image.
Add a background layer using a color that will make your main image pop out of the poster. Use Photoshop's Text Tool to add text to your poster. Design the tagline and title in a font and size where they will be easy to read from far off and do not obscure your main image. Add the credits at the bottom in a much smaller and lighter font. Look at existing movie posters for the correct order in which to create your credits.