Download the file
Move the file to /usr/share/vim/vimfiles/
sudo mv plugin.zip /usr/share/vim/vimfiles/
Extract the file
sudo unzip plugin.zip
sudo tar -xvwzf plugin.tar.gz
If this ends up creating a directory instead of extracting the files, do the following steps.
cd into the folder
Move its contents
sudo mv ./ ..
You can now start using the plugin. But beware, for many plugins you are also required to edit the vim configuration file at /etc/vim/vimconfig
My Clojure plugin needed syntax highlighting and and filetype plugin indent on. Your README file (if there is one) should tell you what to do.
Download the plugin
Run a file manager as root.
This is usually just a matter of uncommenting a few lines. I had to uncomment syntax highlighting and file auto indent.