Open a website which specializes in movie trailers, such as apple.com/trailers, movie-list.com/trailers, or empiremovies.com/trailers/.
Type in the title of the movie into the search engine in the site.
Determine the size you want your trailer to be. Most trailer sites offer high-res, medium-res and low-res copies. High-res downloads tend to use more space on your hard drive. If you want to download a lot of trailers, consider using a lower resolution.
Check to make sure your computer has software which supports the trailer's format. Most computers have Quicktime or Windows Media, which are the basics for these kind of downloads. Be sure you've selected the format you need.
Click on the download link to download the trailer.
Open the downloaded file using the software from Step 4.