A highly customizable widget that allows you to display the top authors of your website easily.
Installing this plugin automatically is the easiest option. You can install the plugin automatically by going to the plugins section in WordPress and clicking Add New. Type Top Authors" in the search bar and install the plugin by clicking the Install Now button.
To manually install the plugin you'll need to download the plugin to your computer and upload it to your server via FTP or another method. The plugin needs to be extracted in the
wp-content/plugins folder. Once done you should be able to activate it as usual.
If you are having trouble, take a look at the Managing Plugins section in the WordPress Codex, it has more information on this topic.