Select the pages, strips or individual panels you'll highlight in the trailer based not only on quality--because in addition to showing your best work--you want it be representative of your story. After all, you could have a great one-off panel that you did on a lark, but if it doesn't fit the rest of your story or series, viewers could get the wrong idea.
Convert those images to a digital format by scanning them in or taking high-resolution photos. Of course, if you create your comics digitally, then you can skip this step.
Plan out your trailer story-board style to coordinate your images, effects, text and any music or voice-overs you want to include. Having a game-plan before getting into the nuts and bolts of putting it together will streamline the process when you start building the trailer on the computer.
Import your images into a computer program that helps you make slide shows or movies. Most computers these days come with basic software for this and each will have their own way of doing things, so run through whatever tutorials came with the software for basic instructions.
Allow enough time on each panel to make sure that viewers have time to read any words inside. The general rule is 1 second per word, but this can be adjusted as necessary to prevent the video from dragging. Voice-overs are another option with a lot of potential.
Use effects sparingly and only if they suit the tone of your comic. Just because you have countless options for transitions, lighting, wave effects and the like doesn't mean you have to use them. Let your artwork and story telling entice the reader, not your special effects.
Introduce characters and plot lines and then direct them to your website or stores that carry your comics.
Add music to your trailer that fits the theme or mood of your comic, but be sure you have permission to use it! There are several sites that have royalty-free music available for free or a nominal fee that you can use without worry of copyright infringement.
Keep it brief. A well-made trailer can be as short as 30 seconds and get the point across, therefore 2 minutes should be your upper limit.
Share your trailer by putting it on your website for your current readers to pass around, as well as upload it to various video-sharing sites on the Internet. Linking to it on various social networking sites is also a good way to get the word out.