Narrow your vision to focus on a smaller chunk of a buffer.
Install it however you usually install plugins.
For example, if you use Vim-Plug, simply add to your vimrc:
Plug 'misterbuckley/vim-tunnel-vision'
Pathogen users:
cd ~/.vim/bundle
git clone https://github.com/misterbuckley/vim-tunnel-vision.git
Put mappings such as the following in your .vimrc:
nmap <Leader>t <Plug>(EnterTunnel)
vmap <Leader>t <Plug>(EnterTunnel)
Then, either visually select a block of text to focus on and press <Leader>t or press <Leader>t in normal mode followed by a motion to focus on that block of text.
The block of text you have selected will be copied to a scratch buffer where those lines are all you see, allowing you to focus on that smaller chunk of text.
When you are done editing in this new buffer, hit :w to exit the tunnel. This will copy your newly edited text back to the original buffer that you came from and destroy the scratch buffer.
It's as simple as that.
Normally when exiting the tunnel with :w, your new changes will not immediately be saved in the original buffer. If g:tv_write_immediately is set to a number greater than zero, your changes will immediately be saved in the original buffer (as if you did another :w).