A photo editing software is needed. I use an old version of Corel Paint Shop Pro and/or the latest version of PhotoScape that is absolutely free on the web.
The next thing you need is a good photo that you can cut in half---because that is basically how much you'll lop off to get the 851px x 315px dimensions. I like to choose a photo that has some close up interest or a nice landscape that can stand a good cropping.
The first step is to reduce the width to 851px---easily done in photoscape using the resize/width setting under the edit tab. Then choose the Crop tab in the lower menu and crop freely to 315px height. Move the crop box around until you get the perfect area of your photo. Then all there is to it is to save and upload.
|Voila! Perfect Facebook Cover Photo!|