Visit Charcoal HQ, then:
In your browser console:
$.getScript('https://rawgit.com/j-f1/91a99c3989aa6c38ee1fcabb67a0f428/raw/install-charcoal-scripts.js')
Alternatively, add this bookmarklet:
javascript:void%20function(){$.getScript(%22https://rawgit.com/j-f1/91a99c3989aa6c38ee1fcabb67a0f428/raw/install-charcoal-scripts.js%22)}();
(I’d make this into a link to install via drag-and-drop, but GitHub Gist removes javascript:
links for security reasons)