the changes would be made in the product_info.php, and Admin/categories.php files
if you have got an unmodified shop then you can just drag and drop the files accross, if not then you would need to use an html editor of somekind, and then run the included sql, using something like phpmyadmin
there are instructions on how to install it all, included with the download.
there is a support thread at http://www.oscommerce.com/forums/index.php?showtopic=344580 if you need any help, or run into any problems, there is also a link within the instructions to an example