Let's say the plugin is at a GitHub URL https://github.com/manasthakur/foo.
First get the plugin by either cloning it (git clone https://github.com/manasthakur.foo.git) or simply downloading it as a zip (from its GitHub page).
Adding a plugin in Vim is equivalent to adding the plugin's code properly into its runtimepath (includes the $HOME/.vim directory by default).
For example, if the layout of a plugin foo is as follows:
foo/autoload/foo.vim
foo/plugin/foo.vim
