WordPress offers a simple and easy way to add plugins to your blog. Access Dashboard, click Plugins link, Add New, and search for the plugin or upload it from Upload Plugin.
Next, activate it from the Plugins page and sit back to observe the work in motion.
Not all plugins are so easy to install, but authors and developers of WordPress make the process as easy as possible.
Information to know before installing
There are some things you should know before you start to install WordPress plugins:
Read the readme: Browse the readme files and web pages, as well as any comments on the plugins on the Web site of an author before you install. The more you know before you install, the easier the process.
Download and update: You must be familiar with how to download and update files and how to use the FTP protocol, if required.
Changing files and templates: Knowledge of PHP, HTML, CSS and CHMOD may be necessary.
Recording of changes: If you make any changes to WordPress files and templates, make a note using the comments in the source code before. Make a backup of your website on your desktop to remind you of any additions or changes you made to the files. This will help you spot them in the future if problems arose.
Make frequent backups: Some plugins work independently of the content of your blog, just adding something special to the page. Others require the modification of the contents of the files, including changes to your database. Before installing any plugin that will make dramatic changes, back up your database and files.
There are several versions of WordPress currently available, and different plugins available for the different versions. Be sure and browse through the information carefully to determine if the plugin will work with your version.
Translated from WordPress.org