The most convenient and reliable way of installing a plugin for me is through the File Manager in cPanel. I searched for a necessary plugin in www.wordpress.com plugin directory.
I downloaded the archive with a plugin on my computer. I entered cPanel — > File Manager and found the public_html directory.
In this directory I found folder with my wordpress blog (it was not in the root) and searched there for wp-content directory. In the “wp-content” folder I found the “plugins” folder. If your blog is in the root, just find wp-content — > plugins.
I opened it and uploaded and extracted the plugin archive. (Read “How to upload and extract files in cPanel?”) I returned to my blog and entered admin panel — > plugins — > installed and clicked “Activate” under the name of the plugin I have installed.