Come up with a basic plot for your comic book. It can encompass one issue or be broken down into multiple issues. It doesn't have to be anything too detailed: just a few paragraphs describing the action and major story points.
Break the plot down into multiple scenes and think about how many images it might take to convey that scene properly. Most comic book pages contain between four and eight images (though they can contain fewer or more in order to achieve a specific effect). Comic books usually have either 32 or 64 pages, though if you intend to publish it yourself, you can go with a larger or smaller number of pages.
Divide your comic book plot into individual chapters. Each chapter should comprise a single issue of the comic book, and ideally end on a cliffhanger or similar plot device intended to lead the reader into the next issue. If you have a short plot, you may be able to fit it into a single issue, but knowing where those breaks are will help you organize your plot appropriately.
Write a draft of your comic book script, covering every aspect of the plot from beginning to end. A script should contain a breakdown of every page, describing the images of each panel as well as the dialogue and sound effects. Don't worry about revising it yet; just get it all down on paper.
Let your script sit for a bit, then go back and revise it. Adjust areas which you feel need more polish, reshape the organization to help it flow, and fix parts of the dialogue which don't work as well as they should. Pay particular attention to the amount of dialogue in each panel -- you don't want to have too much or it will overwhelm the image -- and the breaks between one page and the other. If possible, you want changes in scenes to occur at the end of each page, and to make sure the last page of your comic book contains the big cliffhanger.
Give a trusted friend the script and let him or her offer suggestions. If possible, your friend should be a professional editor or someone with a background in English. Regardless, your friend should be honest in pointing out problem areas and helpful in mentioning possible solutions.
Contact an artist to see about drawing your comic book plot.