Open a search engine and enter the search criteria. If you are looking for a general price guide, just type in something along the lines of "Free Comic Book Price Guide Download." However, there are also specific price guides out there if you know exactly which comic books you are looking to buy or sell. For example, if you are interested in Marvel Comics, you can search for a Marvel Comic Book Price Guide.
When you see an appropriate website, click on it. Read the price guide's description to make sure it's exactly what you want, containing not only a correct comic list but also current pricing. Read the system requirements for the download, if there are any, and make sure that your computer is updated enough so that it can support the file.
Click on the "Download" link or button. When you do this, your Internet browser's download manager will pop up. Depending on what your settings are, the file will either download directly into a downloads folder onto your computer's hard drive, or it will ask you what folder you would like the file to be in. If the second option comes up, select a place on your computer where you'll be sure to find the price guide, whether it's your desktop or a separate folder you've created for comic books. After a few minutes of downloading, the file will be on your computer. Now you can open it up and begin pricing out your comics.