By Lisa Sabin-Wilson from WordPress All-in-One For Dummies, 2nd Edition
Installing plugins from the WordPress Dashboard is so easy that you probably never need to know how to install a plugin manually via FTP. But the technique is still helpful to know in case the WordPress Plugin Directory is down or unavailable.

The following steps take you through how to install a plugin using FTP, using the Gallery plugin as the example:

Go to the plugin page from the WordPress Plugin Directory website.

Click the red download button to transfer the plugin Zip file to your computer.

Unzip the plugin files.

All plugins downloaded from the Plugin Directory are in the Zip format. Most operating systems (Windows, Mac, and so on) have built-in tools to open Zip files. After opening the Zip file, extract the directory contained inside the Zip file and put it in a directory on your computer that is easily accessible.

Connect to your site’s server using FTP.

If you have any difficulty connecting to your server, contact your hosting provider and ask for assistance in connecting to your server via FTP.

Navigate to the wp-content folder within the WordPress installation for your website or blog.

The location of your WordPress installation can differ with every hosting provider. Make sure that you know the location before you proceed.

Navigate to the /usr/share/wordpress/wp-content/plugins directory.

First, navigate to wp-content. Inside this directory are the plugins and themes directories along with a few others. Navigate to the plugins directory. It is inside this directory that all plugins reside.

Upload the plugin folder to the /wp-content/plugins directory on your web server.

The plugin folder, named for the plugin (for example, if you’re uploading the Gallery plugin, the folder is /gallery) contains all the files for that plugin.

Go to the Dashboard’s Plugins page and you see the new plugin listed. If a mistake is made, delete all the newly uploaded files and begin again.